Controlled growth of nanoscale wires

ABSTRACT

The present invention generally relates to nanoscale wires, and to methods of producing nanoscale wires. In some aspects, the nanoscale wires are nanowires comprising a core which is continuous and a shell which may be continuous or discontinuous, and/or may have regions having different cross-sectional areas. In some embodiments, the shell regions are produced by passing the shell material (or a precursor thereof) over a core nanoscale wire under conditions in which Plateau-Raleigh crystal growth occurs, which can lead to non-homogenous deposition of the shell material on different regions of the core. The core and the shell each independently may comprise semiconductors, and/or non-semiconductor materials such as semiconductor oxides, metals, polymers, or the like. Other embodiments are generally directed to systems and methods of making or using such nanoscale wires, devices containing such nanoscale wires, or the like.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/989,904, filed May 7, 2014, entitled “ControlledGrowth of Nanoscale Wires,” by Lieber, et al., incorporated herein byreference in its entirety.

GOVERNMENT FUNDING

This invention was made with government support under Grant No.N00244-09-1-0078 awarded by Department of Defense, Office of NavalResearch. The government has certain rights in the invention.

FIELD

The present invention generally relates to nanoscale wires, and tomethods of producing nanoscale wires.

BACKGROUND

Interest in nanotechnology, in particular sub-microelectronictechnologies such as semiconductor quantum dots and nanowires, has beenmotivated by the challenges of chemistry and physics at the nanoscale,and by the prospect of utilizing these structures in electronic andrelated devices. Nanoscopic articles might be well-suited for transportof charge carriers and excitons (e.g. electrons, electron pairs, etc.)and thus may be useful as building blocks in nanoscale applications.

SUMMARY

The present invention generally relates to nanoscale wires, and tomethods of producing nanoscale wires. The subject matter of the presentinvention involves, in some cases, interrelated products, alternativesolutions to a particular problem, and/or a plurality of different usesof one or more systems and/or articles.

In one aspect, the present invention is generally directed to anarticle, such as a nanoscale wire. In some cases, the nanoscale wire isa nanowire. For example, in one set of embodiments, the articlecomprises a nanowire comprising a continuous core comprising a firstlongitudinal portion comprising a shell at least partially surroundingthe core and having a substantially constant cross-sectional area, and asecond longitudinal portion having a substantially constantcross-section area smaller than the first longitudinal portion. In someembodiments, the first longitudinal portion has a first dimensionorthogonal to the core and a second dimension orthogonal to the firstdimension and to the core, where an aspect ratio of the first dimensionto the second dimension is at least about 1.5:1. In certain cases, thecore and the shell material have different compositions.

In another set of embodiments, the article comprises a nanowirecomprising a continuous semiconductor oxide core and a plurality ofdiscontinuous semiconductor shell regions.

The article, in yet another set of embodiments, comprises a nanowirecomprising a continuous semiconductor oxide core comprising a firstlongitudinal portion having a length of at least 5 nm and a firstsubstantially constant cross-sectional area, a second longitudinalportion having a length of at least 5 nm a second substantially constantcross-section area smaller than the first longitudinal portion, and atransitional portion between the first longitudinal portion and thesecond longitudinal portion, the transitional portion having a length ofat least about 10 nm.

According to still another set of embodiments, the article comprises ananowire comprising a continuous core and a plurality of discontinuousshell regions. In some cases, some or all of the discontinuous shellregions each consist of a single crystal.

In one set of embodiments, the article comprises a nanowire comprising acontinuous core and a plurality of discontinuous shell regions. In somecases, the discontinuous shell regions each consist of a single crystal.

The article, in another set of embodiments, includes a nanowirecomprising a continuous metallic core and a plurality of discontinuoussemiconductor shell regions.

In yet another set of embodiments, the article comprises a nanowirecomprising a continuous polymeric core and a plurality of discontinuoussemiconductor shell regions.

According to still another set of embodiments, the article comprises ananowire comprising a continuous core and a plurality of discontinuousshell regions. In some embodiments, the discontinuous shell regions eachcomprise a plurality of nanoparticles.

The article, in another set of embodiments, includes a nanowirecomprising a continuous semiconductor oxide core comprising a firstlongitudinal portion having a length of at least 5 nm and asubstantially constant cross-sectional area, a second longitudinalportion having a length of at least 5 nm and a substantially constantcross-section area smaller than the first longitudinal portion. In someembodiments, the first longitudinal portion has a first dimensionorthogonal to the core and a second dimension orthogonal to the firstdimension and to the core. In certain cases, an aspect ratio of thefirst dimension to the second dimension is at least about 1.5:1.

The article, in yet another set of embodiments, comprises a nanowirecomprising a continuous metallic core comprising a first longitudinalportion having a length of at least 5 nm and a first substantiallyconstant cross-sectional area, a second longitudinal portion having alength of at least 5 nm a second substantially constant cross-sectionarea smaller than the first longitudinal portion, and a transitionalportion between the first longitudinal portion and the secondlongitudinal portion, the transitional portion having a length of atleast about 10 nm.

In still another set of embodiments, the article includes a nanowirecomprising a continuous metallic core comprising a first longitudinalportion having a length of at least 5 nm and a substantially constantcross-sectional area, a second longitudinal portion having a length ofat least 5 nm and a substantially constant cross-section area smallerthan the first longitudinal portion. In some cases, the firstlongitudinal portion has a first dimension orthogonal to the core and asecond dimension orthogonal to the first dimension and to the core,wherein an aspect ratio of the first dimension to the second dimensionis at least about 1.5:1.

According to yet another set of embodiments, the article comprises ananowire comprising a continuous polymeric core comprising a firstlongitudinal portion having a length of at least 5 nm and a firstsubstantially constant cross-sectional area, a second longitudinalportion having a length of at least 5 nm a second substantially constantcross-section area smaller than the first longitudinal portion, and atransitional longitudinal portion between the first longitudinal portionand the second longitudinal portion, the transitional portion having alength of at least about 10 nm.

In another set of embodiments, the article comprises a nanowirecomprising a continuous polymeric core comprising a first longitudinalportion having a length of at least 5 nm and a substantially constantcross-sectional area, a second longitudinal portion having a length ofat least 5 nm and a substantially constant cross-section area smallerthan the first longitudinal portion. In some cases, the firstlongitudinal portion has a first dimension orthogonal to the core and asecond dimension orthogonal to the first dimension and to the core. Insome embodiments, an aspect ratio of the first dimension to the seconddimension is at least about 1.5:1.

In accordance with another set of embodiments, the article includes ananowire comprising a continuous core comprising a first longitudinalportion comprising a shell at least partially surrounding the corehaving a length of at least 5 nm and a substantially constantcross-sectional area, a second longitudinal portion having a length ofat least 5 nm and a substantially constant cross-section area smallerthan the first longitudinal portion, and a transitional longitudinalportion between the first longitudinal portion and the secondlongitudinal portion, where the transitional portion has a length of atleast about 10 nm. In some embodiments, the core and the shell materialcomprise different materials.

The article, in yet another set of embodiments, comprises a nanowirecomprising a continuous core comprising a first longitudinal portionhaving a length of at least 5 nm and a first substantially constantcross-sectional area, a second longitudinal portion having a length ofat least 5 nm a second substantially constant cross-section area smallerthan the first longitudinal portion, and a transitional longitudinalportion between the first longitudinal portion and the secondlongitudinal portion, the transitional longitudinal having a length ofat least about 10 nm.

According to still another set of embodiments, the article comprises ananowire comprising a continuous core comprising a first longitudinalportion having a length of at least 5 nm and a substantially constantcross-sectional area, a second longitudinal portion having a length ofat least 5 nm and a substantially constant cross-section area smallerthan the first longitudinal portion. In some embodiments, the firstlongitudinal portion has a first dimension orthogonal to the core and asecond dimension orthogonal to the first dimension and to the core. Incertain cases, an aspect ratio of the first dimension to the seconddimension is at least about 1.5:1.

In another set of embodiments, the present invention is generallydirected to a nanowire comprising a core and at least one shell, e.g.,as discussed herein. The core may comprise a metal, a semiconductor, asemiconductor oxide, a polymer, particles, or the like. The shell mayindependently be a semiconductor, a metal, a polymer, an oxide, aninsulator, a dielectric, and/or the shell may comprise particles. Thus,for example, the core may comprise a metal and the shell may comprise asemiconductor, or the core may comprise a metal and the shell maycomprise a metal, or the core may comprise a metal and the shell maycomprise a polymer, or the core may comprise a metal and the shell maycomprise a semiconductor oxide, or the core may comprise a metal and theshell may comprise a semiconductor, or the core may comprise a metal andthe shell may comprise an insulator, or the core may comprise a metaland the shell may comprise a dielectric, or the core may comprise ametal and the shell may comprise particles, or the core may comprise asemiconductor and the shell may comprise a semiconductor, or the coremay comprise a semiconductor and the shell may comprise a metal, or thecore may comprise a semiconductor and the shell may comprise a polymer,or the core may comprise a semiconductor and the shell may comprise asemiconductor oxide, or the core may comprise a semiconductor and theshell may comprise a semiconductor, or the core may comprise asemiconductor and the shell may comprise an insulator, or the core maycomprise a semiconductor and the shell may comprise a dielectric, or thecore may comprise a semiconductor and the shell may comprise particles,or the core may comprise a semiconductor oxide and the shell maycomprise a semiconductor, or the core may comprise a semiconductor oxideand the shell may comprise a metal, or the core may comprise asemiconductor oxide and the shell may comprise a polymer, or the coremay comprise a semiconductor oxide and the shell may comprise asemiconductor oxide, or the core may comprise a semiconductor oxide andthe shell may comprise a semiconductor, or the core may comprise asemiconductor oxide and the shell may comprise an insulator, or the coremay comprise a semiconductor oxide and the shell may comprise adielectric, or the core may comprise a semiconductor oxide and the shellmay comprise particles, or the core may comprise a polymer and the shellmay comprise a semiconductor, or the core may comprise a polymer and theshell may comprise a metal, or the core may comprise a polymer and theshell may comprise a polymer, or the core may comprise a polymer and theshell may comprise a semiconductor oxide, or the core may comprise apolymer and the shell may comprise a semiconductor, or the core maycomprise a polymer and the shell may comprise an insulator, or the coremay comprise a polymer and the shell may comprise a dielectric, or thecore may comprise a polymer and the shell may comprise particles, etc.

In addition, the present invention, in some aspects, is generallydirected to systems and methods for making a nanoscale wire. In somecases, the nanoscale wire is a nanowire. In one set of embodiments, forexample, the method includes acts of depositing a shell material on ananowire by flowing a fluid comprising the shell material or a precursorthereof over the nanowire at a flowrate of less than about 10 sccm at atemperature of between about 700° C. and about 900° C. and under apressure of less than about 1 torr. The fluid may be a liquid or a gas(or the fluid may comprise both a liquid and a gas, in some cases).

According to another set of embodiments, the method includes an act ofdepositing a shell material on a nanowire by flowing a fluid comprisingthe shell material or a precursor thereof over the nanowire such thatthe surface diffusion length of the shell material on the surface of thenanowire is at least about 100 nm.

In still another set of embodiments, the method includes an act ofdepositing a shell material on a nanowire by flowing a fluid comprisingthe shell material (or a precursor thereof) over the nanowire at aflowrate of less than about 10 sccm at a temperature of between about700° C. and about 900° C. and under a pressure of less than about 1torr. In some cases, the flowrate may be less than about 20 sccm. Insome cases, the fluid flows longitudinally over the nanowire.

The method, in yet another set of embodiments, includes an act ofdepositing a shell material (or a precursor thereof) on a nanowire byflowing a fluid comprising the shell material over the nanowire suchthat the surface diffusion length of the shell material on the surfaceof the nanowire is at least about 100 nm. In some cases, the fluid flowslongitudinally over the nanowire.

In one set of embodiments, the method includes an act of flowing afluid, such as a liquid and/or a gas, comprising a shell material (or aprecursor thereof) over a nanowire such that the shell material depositson the nanowire in a plurality of discontinuous shell regions. In somecases, the fluid flows longitudinally over the nanowire.

The method, in accordance with another set of embodiments, includes anact of depositing a shell material (or a precursor thereof) on ananowire by flowing a fluid, such as a liquid and/or a gas, comprisingthe shell material over the nanowire under Plateau-Raleigh crystalgrowth conditions. In some cases, the fluid flows longitudinally overthe nanowire.

The method, in still another set of embodiments, includes an act ofdepositing a shell material (or a precursor thereof) on a nanowire byflowing a fluid, such as a liquid and/or a gas, comprising the shellmaterial over the nanowire such that the shell material is able tominimize its surface area. In some cases, the fluid flows longitudinallyover the nanowire.

In yet another set of embodiments, the method includes an act ofthermally evaporating a shell material (or a precursor thereof) onto ananowire such that the shell material deposits on the nanowire in aplurality of discontinuous shell regions. In yet another set ofembodiments, the method includes an act of depositing a shell material(or a precursor thereof) onto a nanowire via physical vapor depositionsuch that the shell material deposits on the nanowire in a plurality ofdiscontinuous shell regions.

According to still another set of embodiments, the method includes anact of depositing a shell material (or a precursor thereof) on ananowire by flowing a fluid comprising the shell material over thenanowire such that the shell material is able to minimize its surfacearea. In some cases, the fluid flows longitudinally over the nanowire.

The method, in yet another set of embodiments, comprises an act ofdepositing a shell material (or a precursor thereof) on a nanowire byflowing a fluid comprising the shell material over the nanowire underPlateau-Raleigh crystal growth conditions. In some cases, the fluidflows longitudinally over the nanowire.

In one set of embodiments, the method includes an act of flowing a fluidcomprising a shell material (or a precursor thereof) over a nanowiresuch that the shell material deposits on the nanowire in a plurality ofdiscontinuous shell regions. In some cases, the fluid flowslongitudinally over the nanowire.

In another aspect, the present invention encompasses methods of makingone or more of the embodiments described herein, for example, nanoscalewires. In still another aspect, the present invention encompassesmethods of using one or more of the embodiments described herein, forexample, nanoscale wires.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control. If twoor more documents incorporated by reference include conflicting and/orinconsistent disclosure with respect to each other, then the documenthaving the later effective date shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIGS. 1A-1D illustrate Plateau-Raleigh crystal growth, in certainembodiments of the invention;

FIGS. 2A-2D illustrate control of Plateau-Raleigh crystal growth,according to some embodiments of the invention;

FIGS. 3A-3D illustrate various nanoscale wires, in some embodiments ofthe invention;

FIGS. 4A-4C illustrate optical properties of certain nanoscale wires, incertain embodiments of the invention;

FIG. 5 illustrates a silicon nanoscale wire;

FIGS. 6A-6B illustrate various diameter-modulated nanoscale wires, insome embodiments of the invention;

FIGS. 7A-7B illustrates a time sequence of Plateau-Raleigh crystalgrowth, in some embodiments of the invention;

FIGS. 8A-8D illustrate pitch as a function of flow rate, in someembodiments of the invention;

FIGS. 9A-9D illustrate pitch as a function of temperature, in certainembodiments of the invention;

FIG. 10 is an SEM of a typical nanoscale wire in accordance with oneembodiment of the invention;

FIGS. 11A-11B illustrate surface area ratios in accordance with someembodiments of the invention;

FIGS. 12A-12B illustrate absorption on certain nanoscale wires, in someembodiments of the invention;

FIGS. 13A-13C are schematic diagrams used for modeling, in certainembodiments of the invention; and

FIG. 14 is a schematic diagram of a nanoscale wire comprisingnanoparticles.

DETAILED DESCRIPTION

The present invention generally relates to nanoscale wires, and tomethods of producing nanoscale wires. In some aspects, the nanoscalewires are nanowires comprising a core which is continuous and a shellwhich may be continuous or discontinuous, and/or may have regions havingdifferent cross-sectional areas. In some embodiments, the shell regionsare produced by passing the shell material (or a precursor thereof) overa core nanoscale wire under conditions in which Plateau-Raleigh crystalgrowth occurs, which can lead to non-homogenous deposition of the shellmaterial on different regions of the core. The core and the shell eachindependently may comprise semiconductors, and/or non-semiconductormaterials such as semiconductor oxides, metals, polymers, or the like.Other embodiments are generally directed to systems and methods ofmaking or using such nanoscale wires, devices containing such nanoscalewires, or the like.

Referring now to FIG. 13A, in one aspect, the present invention isgenerally directed to a nanoscale wire 10 having a core 20 and one ormore shell regions 30. The core 20 may be a nanowire, and may comprise asemiconductor material (e.g., germanium, silicon, indium phosphide,etc.), a dielectric material such as a semiconductor oxide (e.g.,silicon dioxide), a metal (e.g., Ni, Pt, Au, etc.), a polymer (e.g.,polyaniline, polypyrrole, etc.), or the like. Other materials arediscussed below. In some embodiments, the core has a substantiallyuniform cross-sectional area along the length of the core. The core canbe prepared using techniques known to those of ordinary skill in theart, such as those discussed below.

There may also be a shell 30 surrounding at least part of the core 20.The shell may be formed from a semiconductor material, or othermaterials such as dielectric materials, semiconductor oxides, metals,polymers, nanoparticles, or the like. In some cases, the shell materialis substantially crystalline, and in some cases, the shell material issubstantially monocrystalline. The core may also be composed ofcrystallites in certain cases. The shell may have the same or adifferent composition from than the core.

The shell may be present as discontinuous regions along core 20, asshown in FIG. 13A, or as a continuous region, but with portions havingdifferent cross-sectional areas. The shell may be cylindricallysymmetrical around the core, as is shown in FIG. 13A, although in somecases, the shell may be non-cylindrically symmetrically distributedaround the core, for example, as shown in FIG. 3D with the shell havinga substantially rectangular cross-sectional area. In certainembodiments, the shell regions may be substantially regularly spacedalong the core 20, and the average spacing or periodicity may bereferred to as the “pitch” of the shells along the core. Thus, as anon-limiting example, the pitch of the shells along the core may bebetween about 5 nm and about 100 micrometers. In addition, it shouldalso be understood that in reality, there may also be some deviationsfrom perfect periodicity or perfect pitch.

In one set of embodiments, the shell may be grown around the core byflowing or passing the shell material 45, or a precursor thereof, alongthe core, as is shown in FIG. 7B. In some cases, shell material (orprecursor) may be present within a fluid (e.g., a liquid, a gas, plasma,etc.), and in some cases, the fluid may be under relatively lowpressures, as discussed below. As the fluid passes past core 20, some ofthe shell material 45 can deposit or form onto the core. Under certainconditions as discussed herein, instabilities may be created, such as inPlateau-Raleigh crystal growth. Without wishing to be bound by anytheory, it is believed that under certain conditions, certain materialshave a tendency to minimize surface area as they deposit onto asubstrate. For example, under some flow conditions as discussed herein,a shell material depositing or forming onto a nanoscale wire surfacewill deposit or form in a way such that the shell material tries tominimize its surface area. The shell material may thus be formed asdiscontinuous sections, or sections having different cross-sectionalareas 30 on the core 20, as it minimizes its surface area. See also FIG.7A, showing shell deposition on a core as a function of time, or FIG. 2,showing pitches with silicon shells on silicon cores of between about 1to 10 micrometers. Other pitches are also possible, as discussed herein.

It should be understood that Plateau-Rayleigh instability is notsynonymous with Plateau-Rayleigh crystal growth. Generally,Plateau-Rayleigh instability is the transformation of a 1-dimensionalliquid-state object into periodically spaced liquid-state spheres, wherethe pitch is limited to about 4 times the diameter of the originalliquid object. This can be observed, for example, in a stream of waterfrom a faucet that eventually breaks apart into separated droplets. Thisbreaking up into droplets is Plateau-Rayleigh instability.Plateau-Rayleigh crystal growth, however, is the growth of a solid-statecrystal on solid-state objects, such as solid-state 1-dimensionalobjects, where the pitch between these shells is determined by thereaction conditions.

As mentioned, various aspects of the present invention are generallydirected to nanoscale wires having a core and one or more shell regionssurrounding at least a portion of the core. The core is typically ananowire, or other suitable nanoscale wire such as those discussedherein. In one set of embodiments, the core has at least onecross-sectional dimension and, in some embodiments, two orthogonalcross-sectional dimensions (e.g., a diameter) of less than 1 micrometer,less than about 500 nm, less than about 200 nm, less than about 150 nm,less than about 100 nm, less than about 70, less than about 50 nm, lessthan about 20 nm, less than about 10 nm, less than about 5 nm, thanabout 2 nm, or less than about 1 nm. In some embodiments, the core isgenerally cylindrical. In other embodiments, however, other shapes arepossible; for example, the core can be faceted, i.e., the core may havea polygonal cross-section. The cross-section of a core can be anyarbitrary shape, including, but not limited to, circular, square,rectangular, annular, polygonal, or elliptical, and may be a regular oran irregular shape. The core may be solid or hollow. In someembodiments, the core may have a substantially uniform profile orcross-sectional area, or have a variation in average diameter of lessthan about 30%, less than about 25%, less than about 20%, less thanabout 15%, less than about 10%, or less than about 5%.

In some cases, the nanoscale wire forming the core has one dimensionthat is substantially longer than the other dimensions of the nanoscalewire. For example, the nanoscale wire may have a longest dimension thatis at least about 1 micrometer, at least about 3 micrometers, at leastabout 5 micrometers, or at least about 10 micrometers or about 20micrometers in length, and/or the nanoscale wire may have an aspectratio (longest dimension to shortest orthogonal dimension) of greaterthan about 2:1, greater than about 3:1, greater than about 4:1, greaterthan about 5:1, greater than about 10:1, greater than about 25:1,greater than about 50:1, greater than about 75:1, greater than about100:1, greater than about 150:1, greater than about 250:1, greater thanabout 500:1, greater than about 750:1, or greater than about 1000:1 ormore in some cases.

The core may be formed out of any of a wide variety of materials. Forinstance, in one set of embodiments, the core may comprise or consistessentially of a semiconductor material. However, it should beunderstood that the core can comprise other materials as well in otherembodiments of the invention. For example, in one set of embodiments,the core may comprise or consist essentially of a metal. In some cases,at least about 80%, at least about 85%, at least about 90%, at leastabout 95%, or 100% of the core (by mass) is a metal. Non-limitingexamples of potentially suitable metals include aluminum, gold, silver,copper, molybdenum, tantalum, titanium, nickel, tungsten, chromium,platinum, or palladium. Techniques for producing metal nanoscale wiresare known to those of ordinary skill in the art, and include, forinstance solution processing techniques such as solution-phasesynthesis, template fabrication techniques, chemical vapor deposition(CVD), or the like. In some cases, metal nanowires may be obtainedcommercially. The core may include one or more than one metals (e.g.,alloyed together).

In another set of embodiments, the core may comprise or consistessentially of a dielectric material. For example, the core may comprisea nitride, such as Si₃N₄, or the core may comprise an oxide, such as asemiconductor oxide or a metal oxide. In one embodiment, thesemiconductor oxide is SiO₂. In another embodiment, the semiconductoroxide is GeO₂. In still other embodiments, the oxide may be SeO₂, SnO₂,GaO₂, TiO₂, Al₂O₃, HfO₂, NiO₂, NiO, BaTiO₃, SrTiO₃, Fe₃O₄, Fe₂O₃, MgO,Cr₂O₃, ZnO, MgO, VO₂, V₂O₅, MnO, CO₂O₃, CO₃O₄, CuO, Cu₂O, ZrO₂, BaO,WO₂, CeO₂, or the like. The core may also comprise other dielectricmaterials, such as NdFeB, or any other suitable material that isdielectric. Combinations of any of these are also possible in somecases, e.g., the semiconductor oxide may comprise SiO₂ and GeO₂, SiO₂and SeO₂, etc. In some cases, at least about 80%, at least about 85%, atleast about 90%, at least about 95%, or 100% of the core (by mass) is asemiconductor oxide. Techniques for producing semiconductor oxidenanoscale wires that can be used as a core will be known to those ofordinary skill in the art, and include, for instance, solutionprocessing techniques, template fabrication techniques, chemical vapordeposition (CVD), or the like.

In yet another set of embodiments, the nanoscale wire core may compriseor consist essentially of a polymer. Examples of polymers include, butare not limited to, polyaniline, polypyrrole, polythiophene,polystyrene, polypropylene, polyester, poly(methyl methacrylate),polyacrylamide, DNA, RNA, SU-8, poly(p-phenylene vinylene),poly(vinylchloride), nylon (e.g., nylon 6, nylon 6,6, etc.),polyurethane, silk, polyphosphazene, low density polyethylene, highdensity polyethylene, polypropylene, thermoplastic polyurethanes,polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinylidenechloride, polysiloxane, polyethylene, polytetrafluoroethylene,poly(ethylene terephthalate), poly(ethylene oxide), and/or derivativesthereof, etc. In some cases, at least about 80%, at least about 85%, atleast about 90%, at least about 95%, or 100% of the core (by mass) is apolymer. Polymeric nanoscale wires that can be used as a core can beprepared using techniques known to those of ordinary skill in the art,such as solution processing techniques, template fabrication techniques,chemical polymerization techniques, etching techniques such as ionetching or plasma etching, or by sol-gel chemistry, etc.

In still another set of embodiments, the nanoscale wire core maycomprise or consist essentially of a semiconductor material. Forexample, the nanoscale wire may comprise silicon. In some cases, thenanoscale wire may comprise germanium. Other suitable semiconductormaterials include those discussed herein. In some cases, at least about80%, at least about 85%, at least about 90%, at least about 95%, or 100%of the core (by mass) is a semiconductor. Typically, a semiconductor isan element having semiconductive or semi-metallic properties (i.e.,between metallic and non-metallic properties). Non-limiting examplesinclude elemental semiconductors, such as silicon, gallium, germanium,diamond (carbon), tin, selenium, tellurium, boron, or phosphorous. Inother embodiments, more than one element may be present, for example,gallium arsenide, gallium nitride, indium phosphide, cadmium selenide,etc. Still other examples include a Group II-VI material (which includesat least one member from Group II of the Periodic Table and at least onemember from Group VI, for example, ZnS, ZnSe, ZnSSe, ZnCdS, CdS, orCdSe), or a Group III-V material (which includes at least one memberfrom Group III and at least one member from Group V, for example GaAs,GaP, GaAsP, InAs, InP, AlGaAs, or InAsP).

Any suitable technique may be used to produce a semiconductor nanoscalewire core, including etching techniques such as ion etching or plasmaetching, vapor-liquid-solid (VLS) synthesis, chemical vapor deposition(CVD) techniques, solution-phase synthesis, supercriticalfluid-liquid-solid growth, or techniques such as those disclosed in U.S.Pat. No. 7,211,464 incorporated herein by reference in its entirety. Asanother example, the core may be grown from substantially uniformnanoclusters or particles, e.g., colloid particles. See, e.g., U.S. Pat.No. 7,301,199, issued Nov. 27, 2007, entitled “Nanoscale Wires andRelated Devices,” by Lieber, et al., incorporated herein by reference inits entirety. Other techniques suitable for producing nanoscale wiresare also known to those of ordinary skill in the art.

In certain embodiments, the semiconductor can be undoped or doped (e.g.,p-type or n-type). For example, in one set of embodiments, a nanoscalewire may be a p-type semiconductor nanoscale wire or an n-typesemiconductor nanoscale wire. In some embodiments, a dopant or asemiconductor may include mixtures of Group IV elements, for example, amixture of silicon and carbon, or a mixture of silicon and germanium. Inother embodiments, the dopant or the semiconductor may include a mixtureof a Group III and a Group V element, for example, BN, BP, BAs, AlN,AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, or InSb. Mixturesof these may also be used, for example, a mixture of BN/BP/BAs, orBN/AlP. In other embodiments, the dopants may include alloys of GroupIII and Group V elements. For example, the alloys may include a mixtureof AlGaN, GaPAs, InPAs, GaInN, AlGaInN, GaInAsP, or the like. In otherembodiments, the dopants may also include a mixture of Group II andGroup VI semiconductors. For example, the semiconductor may include ZnO,ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS,MgSe, or the like. Alloys or mixtures of these dopants are also bepossible, for example, (ZnCd)Se, or Zn(SSe), or the like. Additionally,alloys of different groups of semiconductors may also be possible, forexample, a combination of a Group II-Group VI and a Group III-Group Vsemiconductor, for example, (GaAs)_(x)(ZnS)_(1-x). Other examples ofdopants may include combinations of Group IV and Group VI elements, suchas GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, or PbTe. Othersemiconductor mixtures may include a combination of a Group I and aGroup VII, such as CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, or thelike. Other dopant compounds may include different mixtures of theseelements, such as BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃,CuSi₂P₃, Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, (Cu,Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂ and the like.

The doping of the semiconductor to produce a p-type or n-typesemiconductor core may be achieved via bulk-doping in certainembodiments, although in other embodiments, other doping techniques(such as ion implantation) can be used. Many such doping techniques thatcan be used will be familiar to those of ordinary skill in the art,including both bulk doping and surface doping techniques. A bulk-dopedarticle (e.g. an article, or a portion or region of an article) is anarticle for which a dopant is incorporated substantially throughout thecrystalline lattice of the article, as opposed to an article in which adopant is only incorporated in particular regions of the crystal latticeat the atomic scale, for example, only on the surface or exterior. Forexample, some articles are typically doped after the base material isgrown, and thus the dopant only extends a finite distance from thesurface or exterior into the interior of the crystalline lattice. Itshould be understood that “bulk-doped” does not define or reflect aconcentration or amount of doping in a semiconductor, nor does itnecessarily indicate that the doping is uniform. “Heavily doped” and“lightly doped” are terms the meanings of which are clearly understoodby those of ordinary skill in the art. In some embodiments, one or moreregions comprise a single monolayer of atoms (“delta-doping”). Incertain cases, the region may be less than a single monolayer thick (forexample, if some of the atoms within the monolayer are absent). As aspecific example, the regions may be arranged in a layered structurewithin the nanoscale wire, and one or more of the regions can bedelta-doped or partially delta-doped.

In some embodiments, the nanoscale wire core has a conductivity of or ofsimilar magnitude to any semiconductor or any metal. The nanoscale wirecan be formed of suitable materials, e.g., semiconductors, metals, etc.,as well as any suitable combinations thereof. In some cases, thenanoscale wire will have the ability to pass electrical charge, forexample, being electrically conductive. For example, the nanoscale wiremay have a relatively low resistivity, e.g., less than about 10⁻³ Ohm m,less than about 10⁻⁴ Ohm m, less than about 10⁻⁶ Ohm m, or less thanabout 10⁻⁷ Ohm m. The nanoscale wire can, in some embodiments, have aconductance of at least about 1 microsiemens, at least about 3microsiemens, at least about 10 microsiemens, at least about 30microsiemens, or at least about 100 microsiemens.

The nanoscale wire core can be solid or hollow, in various embodiments.As used herein, a “nanotube” is a nanoscale wire that is hollow, or thathas a hollowed-out core, including those nanotubes known to those ofordinary skill in the art. As another example, a nanotube may be createdby creating a core/shell nanowire, then etching away at least a portionof the core to leave behind a hollow shell. Accordingly, in one set ofembodiments, the nanoscale wire is a non-carbon nanotube. In contrast, a“nanowire” is a nanoscale wire that is typically solid (i.e., nothollow). Thus, in one set of embodiments, the nanoscale wire may be asemiconductor nanowire, such as a silicon nanowire.

In one set of embodiments, the nanoscale wire may include aheterojunction, e.g., of two regions with dissimilar materials orelements, and/or the same materials or elements but at different ratiosor concentrations. The regions of the nanoscale wire may be distinctfrom each other with minimal cross-contamination, or the composition ofthe nanoscale wire can vary gradually from one region to the next. Theregions may be both longitudinally arranged relative to each other, orradially arranged (e.g., as in a core/shell arrangement) on thenanoscale wire. Each region may be of any size or shape within the wire.The junctions may be, for example, a p/n junction, a p/p junction, ann/n junction, a p/i junction (where i refers to an intrinsicsemiconductor), an n/i junction, an i/i junction, or the like. Thejunction can also be a Schottky junction in some embodiments. Thejunction may also be, for example, a semiconductor/semiconductorjunction, a semiconductor/metal junction, a semiconductor/insulatorjunction, a metal/metal junction, a metal/insulator junction, aninsulator/insulator junction, or the like. The junction may also be ajunction of two materials, a doped semiconductor to a doped or anundoped semiconductor, or a junction between regions having differentdopant concentrations. The junction can also be a defected region to aperfect single crystal, an amorphous region to a crystal, a crystal toanother crystal, an amorphous region to another amorphous region, adefected region to another defected region, an amorphous region to adefected region, or the like. More than two regions may be present, andthese regions may have unique compositions or may comprise the samecompositions. As one example, a wire can have a first region having afirst composition, a second region having a second composition, and athird region having a third composition or the same composition as thefirst composition. Non-limiting examples of nanoscale wires comprisingheterojunctions (including core/shell heterojunctions, longitudinalheterojunctions, etc., as well as combinations thereof) are discussed inU.S. Pat. No. 7,301,199, issued Nov. 27, 2007, entitled “Nanoscale Wiresand Related Devices,” by Lieber, et al., incorporated herein byreference in its entirety.

In one set of embodiments, the nanoscale wire core is formed from asingle crystal, for example, a single crystal nanoscale wire comprisinga semiconductor. A single crystal item may be formed via covalentbonding, ionic bonding, or the like, and/or combinations thereof. Whilesuch a single crystal item may include defects in the crystal in somecases, the single crystal item is distinguished from an item thatincludes one or more crystals, not ionically or covalently bonded, butmerely in close proximity to one another. The single crystal may be onethat contains no grain boundaries, although it may contain defects,dislocations, impurities, etc. in some cases. However, in otherembodiments, the core may be composed of crystallites, the core may bepolycrystalline or single crystalline, etc.

In some embodiments, the nanoscale wires used herein are individual orfree-standing nanoscale wires. For example, an “individual” or a“free-standing” nanoscale wire may, at some point in its life, not beattached to another article, for example, with another nanoscale wire,or the free-standing nanoscale wire may be in solution. This is incontrast to nanoscale features etched onto the surface of a substrate,e.g., a silicon wafer, in which the nanoscale features are never removedfrom the surface of the substrate as a free-standing article. This isalso in contrast to conductive portions of articles which differ fromsurrounding material only by having been altered chemically orphysically, in situ, i.e., where a portion of a uniform article is madedifferent from its surroundings by selective doping, etching, etc. An“individual” or a “free-standing” nanoscale wire is one that can be (butneed not be) removed from the location where it is made, as anindividual article, and transported to a different location and combinedwith different components to make a functional device such as thosedescribed herein and those that would be contemplated by those ofordinary skill in the art upon reading this disclosure.

The core may be surrounded by one or more shell materials. These shellmaterials may be deposited or formed onto the core, in one aspect of theinvention, by deposition techniques where a fluid, such as a liquidand/or a gas, flows or passes by a core, e.g., in a longitudinaldirection relative to the core, as is shown in FIG. 7B. Some of theshell material (or a precursor of the shell material) can deposit on thecore, e.g., to form one or more shells surrounding at least a portion ofthe core. This deposition may not necessarily occur uniformly. Rather,the deposition of the shell materials or precursors may be performedunder conditions that cause nonuniform deposition or formation of theshell materials. For instance, the shell materials may deposit or formas a series of discontinuous regions separated by regions of the corethat do not contain any deposits (or any substantial deposits) of theshell material. In some cases, the shell materials or precursors maydeposit such that the surface area is relatively minimized and the shellmaterial is concentrated or located in certain regions along the core.For instance, as discussed in more detail below, reaction conditions maybe applied to create conditions in which the shell material seeks tominimize surface area, e.g., such that Plateau-Rayleigh crystal growthoccurs.

The shell regions may be discontinuous and separated along the nanoscalewire in certain embodiments. For instance, one shell region may beseparated from its next nearest shell region by a distance along thecore of at least about 1 nm, at least about 3 nm, at least about 5 nm,at least about 7 nm, at least about 10 nm, at least about 15 nm, atleast about 30 nm, at least about 50 nm, at least about 75 nm, at leastabout 100 nm, at least about 200 nm, at least about 300 nm, at leastabout 500 nm, at least about 750 nm, at least about 1 micrometer, atleast about 2 micrometers, at least about 3 micrometers, at least about4 micrometers, at least about 5 micrometers, at least about 6micrometers, at least about 7 micrometers, at least about 8 micrometers,at least about 10 micrometers, at least about 15 micrometers, at leastabout 20 micrometers, at least about 30 micrometers, at least about 50micrometers, at least about 75 micrometers, or more in some cases. Insome cases, the distance of separation may be no more than about 100micrometers, no more than about 75 micrometers, no more than about 50micrometers, no more than about 30 micrometers, no more than about 20micrometers, no more than about 15 micrometers, no more than about 10micrometers, no more than about 8 micrometers, no more than about 7micrometers, no more than about 6 micrometers, no more than about 5micrometers, no more than about 4 micrometers, no more than about 3micrometers, no more than about 2 micrometers, no more than about 1micrometer, no more than about 750 nm, no more than about 500 nm, nomore than about 300 nm, no more than about 200 nm, no more than about100 nm, no more than about 75 nm, no more than about 50 nm, no more thanabout 30 nm, no more than about 15 nm, no more than about 10 nm, no morethan about 7 nm, no more than about 5 nm, or no more than about 3 nm.Combinations of any of these are also possible; for instance, two shellregions may be separated by a distance of separation between about 1micrometer and about 5 micrometers.

In some cases, the shells may be substantially regularly spaced alongthe core. It should be understood that in reality, the spacing may notnecessarily be perfect, but there may be some variation in the spacing.The average spacing between shells may be referred to as the “pitch” ofthe shells, and is usually measured from the leading edge of one shellregion to the leading edge of the next shell region (see, e.g., FIG.1A). Multiple such distances may also be averaged together to determinethe average pitch. In some embodiments, the pitch may fall within any ofthe ranges given above for the distance of separation, e.g., in one setof embodiments, the pitch may be between about 1 micrometer and about 5micrometers. However, as mentioned, it should be understood that theaverage spacing or pitch is an average, and individual shell regions maybe closer or farther away. For instance, there may be a standarddeviation or variation of at least about 3%, at least about 5%, at leastabout 10%, at least about 15%, at least about 20%, at least about 25% ofthe mean value of the pitch.

These shell regions may be spherical in some embodiments (which wouldrepresent an idealized minimal state), but in other embodiments, theregions can be non-spherical. For instance, in some cases, the shellmaterial (or precursor thereof) may deposit as a crystal, and in somecases, the depositions may be monocrystalline or substantiallymonocrystalline, e.g., without discrete domains or grain boundarieswithin a single shell region. Thus, in some embodiments, a shell regionis a single crystal. In some cases, due to the crystallinity, theregions may deposit non-spherically. For example, as is shown in FIG.3D, the regions may deposit with generally square or rectangularcross-sectional areas, or with areas having an aspect ratio (i.e., oftwo dimensions orthogonal to the longitudinal direction) of at leastabout 1.3:1, at least about 1.5:1, at least about 1.8:1, at least about2:1, at least about 2.5:1, at least about 3:1, at least about 4:1, atleast about 5:1, etc.

In some cases, the shell regions comprise a portion that has asubstantially uniform cross-sectional area, relative to the longitudinaldirection along the nanoscale wire core. Optionally, the regions mayalso comprise portions that do not have a uniform cross-sectional area,which can be understood to be a transition portion, e.g., as shown in across-sectional view in FIG. 13B. In some embodiments, a shell regionmay have a portion having a uniform cross-sectional area having amaximum dimension (orthogonal to the core) of greater than about 10 nm,greater than about 30 nm, greater than about 50 nm, greater than about75 nm, greater than about 100 nm, greater than about 150 nm, greaterthan about 200 nm, greater than about 250 nm, greater than about 300 nm,greater than about 350 nm, greater than about 400 nm, greater than about450 nm, greater than about 500 nm, greater than about 600 nm, greaterthan about 700 nm, greater than about 800 nm, greater than about 900 nm,greater than about 1 micrometer, greater than about 2 micrometers,greater than about 3 micrometers, greater than about 4 micrometers, orgreater than about 5 micrometers. In addition, in some cases, thedimension may be less than about 5 micrometers, less than about 4micrometers, less than about 3 micrometers, less than about 2micrometers, less than about 1 micrometer, less than about 900 nm, lessthan about 800 nm, less than about 700 nm, less than about 600 nm, lessthan about 500 nm, less than about 450 nm, less than about 400 nm, lessthan about 350 nm, less than about 300 nm, less than about 250 nm, lessthan about 200 nm, less than about 100 nm, less than about 75 nm, lessthan about 50 nm, less than about 30 nm, less than about 10 nm, etc.Combinations of any of these are also possible in some embodiments; forinstance, a shell region may have a dimension orthogonal to the core ofbetween about 100 nm and about 400 nm. In addition, as mentioned, theshell material may also have a substantially circular cross-sectionalarea, or have other geometries or other various aspect ratios.Dimensions of the core are discussed elsewhere herein, and can becombined with any of these dimensions (or other dimensions as describedherein) of the shell, in certain embodiments.

The shell regions may also have a length determined longitudinally alongthe core of at least about 1 nm, at least about 3 nm, at least about 5nm, at least about 7 nm, at least about 10 nm, at least about 15 nm, atleast about 30 nm, at least about 50 nm, at least about 75 nm, at leastabout 100 nm, at least about 200 nm, at least about 300 nm, at leastabout 500 nm, at least about 750 nm, at least about 1 micrometer, atleast about 2 micrometers, at least about 3 micrometers, at least about4 micrometers, at least about 5 micrometers, at least about 6micrometers, at least about 7 micrometers, at least about 8 micrometers,at least about 10 micrometers, at least about 15 micrometers, or more insome cases. In some cases, the length may be no more than about 20micrometers, no more than about 15 micrometers, no more than about 10micrometers, no more than about 8 micrometers, no more than about 7micrometers, no more than about 6 micrometers, no more than about 5micrometers, no more than about 4 micrometers, no more than about 3micrometers, no more than about 2 micrometers, no more than about 1micrometer, no more than about 750 nm, no more than about 500 nm, nomore than about 300 nm, no more than about 200 nm, no more than about100 nm, no more than about 75 nm, no more than about 50 nm, no more thanabout 30 nm, no more than about 15 nm, no more than about 10 nm, no morethan about 7 nm, no more than about 5 nm, or no more than about 3 nm.Combinations of any of these are also possible. For example, in oneembodiment, the shell may have a length of between about 1 micrometerand about 2 micrometers.

In certain embodiments, the nanoscale wire (including the core andshell) has a maximum dimension, orthogonal to the core, of less thanabout 5 micrometers, less than about 4 micrometers, less than about 3micrometers, less than about 2 micrometers, less than about 1micrometer, less than about 900 nm, less than about 800 nm, less thanabout 700 nm, less than about 600 nm, less than about 500 nm, less thanabout 450 nm, less than about 400 nm, less than about 350 nm, less thanabout 300 nm, less than about 250 nm, less than about 100 nm, less thanabout 75 nm, less than about 50 nm, less than about 30 nm, less thanabout 10 nm, etc.

In some embodiments, the shell region may also contain one or moretransition portions, e.g., regions that do not have a uniformcross-sectional area. Typically, the transition portion may bepositioned between the shell region and regions of the nanoscale wirecore that are substantially free of such shell portions (or have smallershell portions). In some cases, the transition portion may have alength, determined along the core, of at least about 5 nm, at leastabout 10 nm, at least about 20 nm, at least about 30 nm, at least about50 nm, at least about 100 nm, etc. In addition, in certain embodiments,the transition portion may have a length of no more than about 100 nm,no more than about 50 nm, no more than about 30 nm, no more than about20 nm, no more than about 10 nm, etc. These may also be combinedtogether in certain embodiments. For example, the transition portion ofthe shell region may have a longitudinal length of between about 10 nmand about 30 nm.

For instance, referring to FIG. 13B, a nanoscale wire may comprise aplurality of periodic shell regions 30 surrounding a core, where theshell regions are separated by regions 37. The shell regions maycomprise a first portion 33 with a substantially cross-sectional area,and a second transition portion 36 that does not have a substantiallycross-sectional area. It should be understood that FIG. 13B is aschematic diagram, and the transition portions may not necessarily beperfectly linear or crystalline. See, e.g., FIG. 1C for examples ofnanoscale wires having transition portions. In addition, in someembodiments, there may not be separate shell regions surrounding thecore, but instead the shell regions may be present as having variationsin thickness, e.g., as is shown in FIG. 13C with first portions 33 andsecond portions 34 around core 20. One or both portions may havesubstantially uniform cross-sectional areas, where first portion 33 islarger than second portion 34. These may also be periodically spaced insome embodiments, e.g., at the pitches described herein.

The shell regions may be formed out of any suitable material, and mayindependently be the same or different from the core. For instance, theshell region may comprise a semiconductor (e.g., silicon, germanium,indium phosphide, etc.), a semiconductor oxide (e.g., silicon dioxide),a metal (e.g., Ni, Pt, Au, etc.), a polymer (e.g., polyaniline,polypyrrole, etc.), or the like. Examples of these and other materialshave been previously described above. In addition, in some cases, theshell region may be present as a single crystal. The shell region mayalso be substantially crystalline, substantially monocrystalline,polycrystalline, single crystalline, or composed of crystallites, etc.In addition, it should be noted that the crystallinity of the core andof the shell may independently be the same or different.

It addition, it should be noted that in some cases, fabricating core andshell regions with different materials results in lattice mismatches,especially where the core and the shell are crystalline ormonocrystalline. In some embodiments, under conditions such asPlateau-Raleigh crystal growth, long surface diffusion length of theshell material on the surface of the nanoscale wire may be required insome embodiments, which could potentially result in failure of the shellmaterial to deposit on the surface of the nanoscale wire. However,surprisingly, it has been found that under some conditions, e.g., withrelatively small shell regions, non-homogenous deposition can stilloccur despite the lattice mismatches.

In addition, in one set of embodiments, the shell region may comprise aplurality of nanoparticles. For instance, as discussed below, in one setof embodiments, a fluid containing nanoparticles may be passed along ananoscale wire, and nanoparticles may deposit onto the nanoscale wire,forming shell regions of nanoparticles around the nanoscale wire, suchas is shown in FIG. 14.

In some cases the nanoparticles may have an average diameter of lessthan about 1 mm, less than about 500 micrometers, less than about 200micrometers, less than about 100 micrometers, less than about 75micrometers, less than about 50 micrometers, less than about 25micrometers, less than about 10 micrometers, or less than about 5micrometers in some cases. The average diameter may also be at leastabout 1 micrometer, at least about 2 micrometers, at least about 3micrometers, at least about 5 micrometers, at least about 10micrometers, at least about 15 micrometers, or at least about 20micrometers in certain cases. The nanoparticles may be, for example,metallic, polymeric, ceramic, or the like. Examples of polymericnanoparticles include, but are not limited to, poly(methylmethacrylate), poly(vinyl alcohol), poly(acrylic acid), polyacrylamide,polymethacrylic acid, polycaprolactone, polylactide, polyglycolide, etc.Examples of metallic nanoparticles include, but are not limited to,gold, silver, copper, platinum, or palladium nanoparticles. Many suchnanoparticles are commercially available.

Another aspect of the present invention is generally directed to systemsand methods of making nanoscale wires as discussed herein. In one set ofembodiments, a fluid containing a shell material (or precursor thereof)is deposited onto a nanoscale wire, or other suitable nanoscale wire.The fluid may be, for example, a liquid, a gas, a plasma, or the like.Combinations of these are also possible, e.g., a mixture of gas andliquid.

A variety of different techniques may be used to flow a material past ananoscale wire, including physical vapor deposition, CVD, thermalevaporation, liquid flow, sputtering, e-beam evaporation, plasma CVD, orthe like. For example, in some embodiments, one or more shell materials(or precursors) are first vaporized into the gaseous phase (e.g., viaheat or temperature, chemical reaction, e-beam evaporation, etc.), thenthe material passes over the nanoscale wire and some of the materialdeposits onto the nanoscale wire. For example, silane (SiH₄) may depositonto the nanoscale wire as Si, germane (GeH₄) may deposit onto thenanoscale wire as Ge, etc. In another set of embodiments, the shellmaterials (or precursors) may be dissolved or suspended in solution, anddeposit onto the surface of the nanoscale wire, e.g., physically orchemically. For example, nanoparticles such as those discussed hereinmay be suspended in aqueous solution then passed over a nanoscale wire.

As a non-limiting example, in one set of embodiments, a shell material(or precursor thereof) may be deposited onto a nanoscale wire usingchemical vapor deposition (CVD). For instance, shell material orprecursor may flow past the nanoscale wire (e.g., in a fluid, and/or invacuum) at a flowrate of less than about 20 sccm, less than about 15sccm, less than about 10 sccm, less than about 5 sccm, etc. In somecases, the temperature may be at least about 600° C., at least about650° C., at least about 700° C., at least about 750° C., at least about800° C., at least about 850° C., at least about 900° C., at least about950° C., etc. In some cases, the temperature may be less than about1000° C., less than about 950° C., less than about 900° C., less thanabout 850° C., less than about 800° C., less than about 750° C., etc.Combinations of these are also possible; for instance, the temperaturemay be between about 700° C. and about 900° C.

In addition, in some cases, the shell material or precursor may bepropelled under vacuum conditions or pressures (e.g., when contained ina fluid). For example, the pressures may be pressures of less than about100 torr (absolute), less than about 50 torr, less than about 30 torr,less than about 10 torr, less than about 5 torr, less than about 3 ton,less than about 1 torr, etc. In some cases, the fluid may comprise gasessuch as H₂, N₂, Ar, He, or Ne.

In one set of embodiments, the shell material or precursor (e.g., in avacuum, or contained in a fluid) is passed or flowed across the coresuch that some of the shell material (or precursor) is able to depositon the core, e.g., to form one or more shell regions. After depositionof the shell material onto the surface, in some cases, there may be somelateral diffusion of the shell material, i.e., on the surface. Undersome conditions, once the shell material (or precursor) deposits ontothe core (or onto deposited shell materials), the shell material (orprecursor) does not immediately become immobilized, but may be able todiffuse on the surface to some extent. For instance, the surfacediffusion length of the shell material on the surface of the nanoscalewire may be at least about 5 nm, at least about 10 nm, at least about 30nm, at least about 50 nm, at least about 100 nm, at least about 300 nm,at least about 500 nm, at least about 750 nm, at least about 1000 nm,etc. The surface diffusion length may be determined using techniquesknown by those of ordinary skill in the art including, for instance,field ion microscopy or scanning tunneling microscopy. In addition, insome embodiments, the surface diffusion length may be estimated using:

${\Lambda = {a\sqrt{\frac{v_{os}n_{o}}{J_{r}}}e^{{{- E_{s}}/2}{RT}}}},$

where lambda (Λ) is the diffusion length, alpha (α) is the adatom hopdistance, or the distance between two neighboring equilbrium positionsof adatoms while they are diffusing, nu (ν_(os)) is a pre-exponentialfrequency factor, n_(o) is the areal density of adsorption sites, J_(r)is the deposition flux of the precursor gas onto the surface, E_(s) isthe activation energy for diffusion, R is the gas constant, and T is thetemperature.

If the surface diffusion length of the shell material is relativelylarge, and/or if the shell material has the ability to diffuse along thesurface of the nanoscale wire, then in some cases, the diffusion of theshell material on the surface of the nanoscale wire (e.g., on the coreand/or the shell material) may occur such that the shell materialreaches a lower energy state where its exposed surface area isminimized. Surprisingly, the minimal state does not occur when the coreis uniformly coated with a homogeneous shell, but instead, the minimalstate is a state where the shell material is concentrated in certainregions (generally spherical) along the core, for instance, where agiven surface area encompasses the largest possible internal volume. Itis surprising that such a phenomenon could be exploited in the nanoscalewire context to produce nanoscale wires having non-uniformly depositedshell, as is discussed herein, or in other solid materials.Plateau-Rayleigh crystal growth has not previously been observed insolids, or in the growth of crystals on a solid object, e.g., on thecore of a nanowire.

Another aspect of the present invention includes the ability tofabricate essentially any electronic device from any of the nanoscalewires discussed herein, for a variety of applications, including but notlimited to electronics, optical, thermal, or mechanical applications.This includes any device that can be made in accordance with this aspectof the invention that one of ordinary skill in the art would desirablymake. Examples of such devices include, but are not limited to, fieldeffect transistors (FETs), bipolar junction transistors (BJTs), tunneldiodes, modulation doped superlattices, complementary inverters, lightemitting devices, light sensing devices, biological system imagers,biological and chemical detectors or sensors, thermal or temperaturedetectors, Josephine junctions, nanoscale light sources, photodetectorssuch as polarization-sensitive photodetectors, gates, inverters, AND,NAND, NOT, OR, TOR, and NOR gates, latches, flip-flops, registers,switches, clock circuitry, static or dynamic memory devices and arrays,state machines, gate arrays, and any other dynamic or sequential logicor other digital devices including programmable circuits. Also includedare analog devices and circuitry, including but not limited to,amplifiers, switches and other analog circuitry using active transistordevices, as well as mixed signal devices and signal processingcircuitry. Also included are p/n junction devices with low turn-onvoltages; p/n junction devices with high turn-on voltages; andcomputational devices such as a half-adder. In some embodiments, thenanoscale wires of the present invention may be manufactured during thedevice fabrication process. In other embodiments, the nanoscale wires ofthe present inventions may first be synthesized, then assembled in adevice.

In some cases, the device may be a nanoscale transistor, such as a fieldeffect transistor (“FET”) or a bipolar junction transistor (“BJT”). Thetransistor may have a smallest width of less than 500 nm, less than 100nm, or other widths as described herein. Any transistor constructedusing adjacent regions having different compositions, are contemplated,for example, arranged longitudinally within a single wire, arrangedradially within the wire, or the like. In one embodiment, a FETcomprising a nanoscale wire may serve as a conducting channel, and anelongated material having a smallest width of less than 500 nm (e.g., ananoscale wire) serving as the gate electrode. For such a FET, thewidths of the nanoscale wire may define a width of the FET. Further, thenanoscale wire may comprise a semiconductor, or have a core/shellarrangement, and such shell may function as a gate dielectric for theFET. In one embodiment, the transistor may be a coaxially-gatedtransistor.

Such distinct nanometer-scale metrics may lead to significantly improveddevice characteristics such as high gain, high speed, and low powerdissipation. Further, such FETs may be readily integratable, and theassembly of such FETs may be shrunk in a straightforward manner intonanometers scale. Such a “bottom-up” approach may scale down to sizesfar beyond what is predicted for traditional “top-down” techniquestypically used in the semiconductor industry today. Further, suchbottom-up assembly may prove to be far cheaper than the traditionaltop-down approach.

Electronic devices incorporating semiconductor nanoscale wires may becontrolled, for example, using any input signal, such as an electrical,optical or a magnetic signal. The control may involve switching betweentwo or more discrete states or may involve continuous control ofnanoscale wire current, i. e., analog control.

The following documents are incorporated herein by reference: U.S. Pat.Nos. 7,129,554, 7,211,464, 7,256,466, 7,301,199, 7,476,596, 7,595,260,7,666,708, 7,915,151, and 8,153,470; Int. Pat. Apl. Pub. Nos. WO02/17362, WO 02/48701, and WO 03/005450; and Int. Pat. Apl. No.PCT/US2014/014596, filed Feb. 4, 2014, entitled “Anisotropic Depositionin Nanoscale Wires,” by Lieber, et al. In addition, U.S. ProvisionalPatent Application Ser. No. 61/989,904, filed May 7, 2014, entitled“Controlled Growth of Nanoscale Wires,” by Lieber, et al. isincorporated herein by reference.

The following examples are intended to illustrate certain embodiments ofthe present invention, but do not exemplify the full scope of theinvention.

Example 1

The Plateau-Rayleigh (PR) instability, as described by Joseph Plateauand Lord Rayleigh in the mid-1800s, describes how a thin column of watercan break apart into droplets to minimize its surface tension. Thefollowing examples describe certain processes using PR instability tocontrol the growth of crystals on 1-dimensional (1D) materials, aprocess termed Plateau-Rayleigh Crystal Growth (PRCG). In theseexamples, Si is deposited onto uniform-diameter Si nanowire (NW) coresto generate diameter-modulated core/shell NW structures. Similarly, Gecan be deposited onto uniform-diameter Ge NW cores. Distinctmorphological features of these structures are broadly tunable throughrational control of reaction conditions for crystalline shell growth.Analysis of the results reveals that an understanding of boththermodynamic/surface energy driving forces and kinetic control ofreaction rates of PR Crystal Growth are necessary to explain the broadrange of modulated structures. To date, the design and synthesis of thevast majority of nanoscale wire structures has relied primarily on twogeneral paradigms: axial modulation during core growth and radialmodulation during shell growth; PR Crystal Growth represents afundamentally new general paradigm for generating diameter-modulatednanoscale wire structures with complex morphologies of various materialsand sizes.

At elevated temperatures, 1D NW cores can serve as starting materialsfor several distinct processes. The introduction of reactive gases canlead to the conformal deposition of crystalline shells as can be seen inFIG. 1A, left. Annealing at low pressures and without reactant gases cantransform various NWs into periodically spaced, isolated crystals due toPR instability (FIG. 1A, top). In contrast to conformal deposition, theintroduction of reactant gases at lower pressures yieldsdiameter-modulated, periodic shell (PS) NWs with several well-definedmorphological features (FIG. 1A, right), including variouscross-sectional aspect ratios (AR, or the ratio of the outer diameter tothe outer height); modulation amplitudes (ratio of outer diameter toinner diameter); and modulation pitches (length of one inner and outershell). Importantly, these morphological features are syntheticallytunable, as they are sensitive to the reaction conditions used for shelldeposition. Although PR instability and PR Crystal Growth both lead toperiodically spaced structures starting from 1D materials, they aredistinct phenomena; traditional PR instability describes constant volumetransformations (i.e. not growth processes) and does not affordtunability in morphology.

The following examples illustrate conformal core/shell growth, PRinstability and PRCG. For these examples, Si NW cores with diameters of100 nm and lengths of 10 to 60 micrometers were first grown via aAu-catalyzed VLS process by chemical vapor deposition (CVD) at 465° C.See, e.g., U.S. Pat. No. 7,211,464; 7,301,199; 7,476,596; 7,595,260;7,666,708; 7,915,151; or 8,153,470, each incorporated herein byreference. For the first growth, shell growth at 775° C., with a SiH₄partial pressure of ˜100 mtorr, and a H₂ partial pressure of ˜25 torrfor ˜15 minutes, yielded highly-crystalline, axially-uniform core/shellnanoscale wires. In a second experiment, after core growth, thetemperature was increased to >775° C. and the total pressure was reducedto ˜0.1 torr. SEM images (FIG. 1B, left) revealed that the corestransformed into particles. The time scales for transformation dependedon temperature and core diameter. At 775° C., 100 nm cores began tobreak up after >14 hours (FIG. 5); at 900° C., the 1D to 0Dtransformation was complete within 3 minutes (FIG. 1B, left). FIG. 5shows an annealed 100 nm Si core NW. SEM image of a representative 100nm Si core NW that was annealed at 775° C. for ˜14 hours at −0.2 torrdirectly following core growth. Scale bar, 200 nm. In a third synthesis,following the growth of 100 nm cores, a SiH₄ partial pressure of ˜1mtorr was introduced at 775° C. for ˜15 minutes. Head-on SEM images(FIG. 1B, right) revealed NWs with low aspect ratios and diametermodulations that periodically repeat along the axis of the NWs.

To demonstrate the tunability of PRCG, separate core/shell syntheseswere performed in which the cores were the same size and grown in thesame manner; however, the temperature and/or partial pressure of SiH₄used during shell growth was varied. Plan-view SEM images (FIG. 1C) ofNWs transferred to Si₃N₄-coated Si substrates showed that the pitch forthese diameter modulations strongly depended on shell reactionconditions. It was noted that the yield for observingdiameter-modulations on a NW for a given synthesis was ˜90% asdetermined by analyses from optical and SEM images (FIG. 6).

FIG. 6 shows yield of diameter-modulated NWs. FIG. 6A shows, left, SEMimage of an approximately 300×250 micrometer area from a PS NW growthsubstrate. Higher-magnification SEM images of NWs denoted by stars areshown on the right. Scale bar, 10 micrometers. Right, high resolutionSEM images of the single NWs denoted in the wide area image to the left.Stars 1 and 3 indicate representative images of NWs that clearly havediameter modulations; Star 2 indicates a NW without clear diametermodulation from SEM imaging. Examining all of the NWs in the large arearegion indicates that the yield of diameter-modulated NWs is ˜90%. Scalebars, 1 micrometers. FIG. 6B shows optical dark field image of a PS NWgrowth substrate. The image was recorded using a 20× objective and anextra 1.6× magnifying lens; subsequently, the color tone of the entireimage was adjusted in Adobe Photoshop to reduce the background in orderto provide the best contrast for the diameter-modulation of the NWs tobe visible.

The PSs were characterized according to their material composition,crystalline quality, and axial uniformity. A low-magnification TEM image(FIG. 1D) showed a PS NW from a synthesis that yields low (˜1) aspectratios with pitch ˜2 micrometers. Close inspection of the interfacebetween one inner and outer shell by bright field (FIG. 1D, left) anddark field (FIG. 1D, middle) imaging did not indicate any appreciabledefects. Electron diffraction obtained from various positions along thenanowire axis (i.e. at the inner shell; outer shell; and the interfacebetween an inner and outer shell) produced indistinguishable diffractionpatterns with a single set of spots in the [111] zone axis (FIG. 1D,right) that indicated a [211] nanowire growth axis. Elemental mapping byEDS and EELS at various axial positions failed to reveal the presence ofany appreciable impurities, suggesting that the nanowire composition wasuniform along its axis. Taken together, TEM imaging, electrondiffraction, and compositional analyses demonstrated that PS NWs werehighly-crystalline materials with clean core/shell interfaces and wereaxially-uniform with respect to crystallographic features and materialcomposition. FIG. 1 shows PR crystal growth of PS NWs with tunablemorphology. FIG. 1A is a schematic illustrating PR instability,conformal shell growth, and PR Crystal Growth of periodic shells on NWcores at elevated temperatures. Inset: definition of syntheticallytunable morphological features: (1) modulation amplitude—ratio of outershell diameter, pouter, to inner shell diameter, D_(inner), (2) aspectratio—ratio of outer shell diameter, D_(outer), to outer shell height,H_(outer) and; (3) pitch—sum of a single outer shell length, L_(outer),and a single inner shell length, L_(inner). FIG. 1B, left, is an SEMimage of Si particles obtained after annealing 100 nm Si NW cores invacuum at 900° C. for 3 minutes. Scale bar, 2 micrometers. Right, end-onSEM image of a typical diameter-modulated core/shell NW. Scale bar, 1micrometer. FIG. 1C is a plan view SEM images showing tunability ofpitch for shells deposited on 100 nm NW cores. The different pitcheswere obtained from distinct syntheses where shell growth temperatureand/or SiH₄ flow rates were changed. Scale bars, 1 micrometer. FIG. 1Dshows crystallographic characterization of PS NWs. Top, compositebright-field TEM image of a low (˜1) aspect ratio Si PS NW with averagepitch of ˜2.2 micrometer. The image shows four individual lowmagnification images stitched together; image borders are indicated bydashed lines. Scale bar, 1 micrometer. Bottom left and middle, brightand dark field TEM images from the area indicated by the arrow. Scalebars, 400 nm. The dark field TEM image was recorded in the [220] zoneaxis. Inset, schematic depicting the cross-sectional geometry andsurface facet assignments of the PS RWs with AR ˜1. Bottom right,electron diffraction patterns in the [111] zone axis from the regionshown to the left.

Example 2

To better understand the mechanism of PS growth, this exampleillustrates a series of growth studies where the core size and synthesiswere kept constant but shell deposition conditions were systematicallychanged. Specifically, Si shells were deposited on 100 nm NW cores fordifferent times, temperatures and SiH₄ flow rates. Plan view SEM imagingwas employed to determine the average pitch for a given synthesis aftertransferring the as-grown PSs onto Si₃N₄-coated Si substrates. SEMimages of PSs grown on 100 nm cores for t=0, 6, and 8 minutes (FIG. 7A)at 800° C. with SiH₄ flow rate of 0.8 sccm illustrated the spontaneousdevelopment of a diameter-modulated shell from a uniform diameter coreNW and that the pitch did not change with time. FIG. 7A shows atime-sequence of PS growth. SEM images of NWs obtained from distinctsyntheses for which Si shells were deposited onto 100 nm cores (at 800°C. with SiH₄ flow rate=0.8 sccm) for times of 0 (left), 6 (middle), and8 (right) minutes. Scale bars, 1 micrometer.

In a second set of experiments, different SiH₄ flow rates were used forfour separate syntheses (FIG. 2A) for a shell deposition temperature of775° C.; plan-view SEM images (FIG. 8) revealed that SiH₄ flow rates of9, 3, 1, and 0.3 sccm yield average pitches of 1.2 micrometers, 2.5micrometers, 3.9 micrometers, and 8.0 micrometers. For a third set ofexperiments, different shell growth temperatures for a given flow rateof SiH₄ (3 sccm) were used for shell growth (FIG. 2B). Deposition at735° C., 775° C., 815° C., and 855° C. yielded average pitches of 1.2micrometers, 2.5 micrometers, 3.7 micrometers, and 8.3 micrometers,respectively, as revealed by multiple pitch measurements from plan-viewSEM images (FIG. 9). At temperatures lower than 700° C., the shells werenot cleanly faceted; at temperatures greater than 900° C., the 100 nmcores underwent a transformation into particles within 3 minutes due toPR instability (FIG. 1B, left). From these systematic growth studies, itcan be concluded that, first, the pitch did not change with total growthtime, and that second, longer pitches were obtained with highertemperatures (lower SiH₄ flow rates) for a given SiH₄ flow rate(temperature).

FIG. 2 shows experimental synthetic control and model for PR CrystalGrowth. FIG. 2A shows the dependence of pitch on SiH₄ flow rate. Sishells were deposited on 100 nm Si cores at 775° C. for various SiH₄flow rates. Error bars denote 1 standard deviation from the average ofat least 10 NWs. FIG. 2B shows the dependence of pitch on temperature.Si shells were deposited on 100 nm Si cores with a SiH₄ flow rate of 3sccm. Error bars denote 1 standard deviation from the average of atleast 10 NWs. FIG. 8 shows pitch as a function of SiH₄ flow rate,including plan view SEM images of PSs grown at ˜0.2 torr total pressureand 775° C. with (FIG. 8A) 0.3, (FIG. 8B) 1, (FIG. 8C) 3, and (FIG. 8D)9 sccm SiH₄. Scale bars, 1 micrometer. FIG. 9 shows pitch as a functionof temperature, including plan view SEM images of PSs grown at ˜0.2 torrtotal pressure with 3 sccm SiH₄ at (FIG. 9A) 735, (FIG. 9B) 775, (FIG.9C) 815, and (FIG. 9D) 855° C. Scale bars, 2 micrometers.

Example 3

Based on the characterization and systematic growth studies, as well asthe substantial theoretical framework of PR instability, this examplepresents a basic model for PR Crystal Growth. However, it should beunderstood that this example is for explanatory purposes only, and thatthe invention as claimed should not be limited to the model provided inthis example.

As the driving force for PR instability is a thermodynamic reduction insurface energy for a fixed amount of volume it is proposed, first, thatthe surface energy of a PS NW is lower than the surface energy of astraight NW with equivalent volume and, second, that NWs with longerpitches have less surface energy than NWs with shorter pitches andequivalent volume. It is further proposed that these longer pitch (i.e.lower surface energy) configurations have higher activation barriers toformation than those with shorter pitches and that the reaction kineticsdetermine whether these lower energy configurations are able to form,specifically by enabling longer Si adatom diffusion lengths.

The experimental results described above are consistent with this model.First, the structural characterization suggests that PS NWs are highquality materials without detectable defects or impurities that couldlead to periodic nucleation. Second, growth studies indicate that pitchdid not change with time; thus, the initial shell deposition conditionsaffect the kinetics of the growth process. Third, for the growth of thinfilms in general, including Si films, higher temperatures and lowerprecursor fluxes may be directly correlated with longer surfacediffusion lengths, and consequently, lower energy film configurations.As shown in FIGS. 2A and 2B, higher temperatures and lower flow rates ofSiH₄ for shell deposition yielded longer pitches. Fourth, H₂ pressuremay significantly reduce the surface diffusion length of Si adatoms; noPS NWs were observed through the deposition of Si at high (>25 torr) H₂pressures. Comparable Si shell growth rates at higher H₂ pressures of 25torr for the same temperature and gas flow rates did not yield PSstructures, suggesting that H₂ itself plays a direct role in kineticproduct formation by reducing Si adatom surface diffusion lengths. Tofurther test this, separate syntheses were performed during which PH₃and B₂H₆ gases were introduced, which are known to reduce Si adatomsurface diffusion lengths, at various flow rates during shell growth.

Introduction of these gases at low flow rates reduced the pitch comparedto standard PR Crystal Growth; at higher flow rates, diameter-modulatedNWs were not observed. These impurities may reduce Si adatom surfacediffusion lengths and thus, kinetically trap shell growth at shorterpitches. PR Crystal Growth may lead to spontaneous growth of periodicshells and that a structural rearrangement (i.e. PR instability) of theshell does not occur. Annealing 100 nm diameter cores at 775° C. for 2hours, which is significantly longer than typical PS growth times of <10minutes, did not lead to significant structural changes to the core,confirming that PR instability itself is not occurring at timescalesrelevant for most PS growths.

In addition to experimental support for this model, a straightforwardgeometric analysis that addresses the thermodynamic aspect of this modelis as follows. For a solid, the total surface energy can be representedas:

$\begin{matrix}{\gamma_{Total} = {\sum\limits_{i}{\gamma_{i}A_{i}}}} & (1)\end{matrix}$

where γ_(Total) (gamma_(Total)) is the total surface energy of thesolid, γ_(i) (gamma_(i)) is the surface energy of facet i, and A_(i) isthe area of surface i. To a first approximation, the relative surfaceenergies of different 1D structures can be reasonably approximated withtheir total SAs, assuming an energetically isotropic solid (see below).The surface areas of various structures can be compared, all of whichhave equivalent volumes. First, the volume of a PS with inner and outershell diameters of 100 and 300 nm with a pitch of 3 μm can be calculated(with input dimensions obtained from the SEM image in FIG. 10); anon-modulated NW with diameter of ˜224 nm has equivalent volume to thisPS. FIG. 10 shows geometrical input for calculations, including an SEMimage of a typical NW whose dimensions were used as inputs for thegeometric simulations in FIGS. 2C and 2D. Scale bar, 1 micrometer. Forthis specific case, the diameter-modulated NW has 0.98 SA of thestraight NW (SA_(straight)). Next, the surface areas of PS NWs areplotted with this same, constant volume and fixed a pitch of 3micrometers but with different modulation amplitudes (FIG. 2C); thedotted line indicates the surface area of the non-modulated (i.e.,straight) NW with a diameter of 224 nm. There is a clear range ofmodulation amplitudes for which a PS has less SA than itsvolume-equivalent straight NW with a minimum value of 0.91 SA_(straight)for a modulation amplitude of ˜4.2.

The SA of PSs of equivalent volume but with different pitches (FIG. 2D)could also be calculated. From this, it was observed that as pitchincreases, the SA decreases. Similar calculations were performed usingvarious fixed total shell volumes for fixed pitch (FIG. 11A) and forvariable pitch (FIG. 11B); while the minimum SA configuration changesfor different shell volumes, the overall trend remains unchanged. FIG.11 shows surface area ratio comparisons with different shell volumes.FIG. 11A, left, the SA PS NW: SA straight NW ratio as a function ofmodulation amplitude for different shell volumes added to a 100 nm core.Right, to-scale 3D PS NW models corresponding to points (I-IV) indicatedon the curve for a given V_(added) and pitch as well as the equivalentvolume straight NW (V). FIG. 11B, left, the minimum SA PS NW: SAstraight NW ratio as a function of pitch for different shell volumesadded to a 100 nm core. Right, to-scale 3D PS NW models corresponding tothe various geometries, all with constant V_(added) but variable pitch,for which the minimum SA for a given V_(added) was calculated along thecurve (I-IV) and the equivalent volume straight NW (V). For both plots,the volumes added to the PS NWs and the straight NWs are equivalent foreach curve. Input geometries were derived from PS NWs withL_(inner)=1500 nm, L_(outer)=1500 nm, D_(inner)=100 nm, and variableD_(outer)=150, 200, 250, 300, 350, and 400 nm (in order from left toright in FIG. 11A, left, and top to bottom, FIG. 11B, right). Forreference, the curves are the same as those shown in FIGS. 3E and 3F.Left/bottom axes, general dimensions rewritten as dimensionless ratios.Absolute SA values, as shown in the top and right axes in FIGS. 3E andF, are different for each plot trace, and thus axes with specific NWdimensions were excluded from this multi-trace plot.

FIG. 2C shows surface area (SA) comparisons for constant pitch. The SAsof PS NWs with various modulation amplitudes are compared to a straightNW with equivalent volume. All structures have equivalent volume and apitch of 3 micrometers. Right/top axes, absolute values from thecalculations assuming total NW length of 30 micrometers and innerdiameter of 100 nm. Left/bottom axes, calculated results rewritten asdimensionless ratios. The SA of a straight NW with equivalent volume isdenoted by the dashed line. All structures have the same total volume.FIG. 2D shows SA comparisons for variable pitch. The SAs of PS NWs withvarious pitches are compared to a straight NW with equivalent volume.All structures have equivalent volume and the lowest possible SAconfiguration for a given pitch. Right/top axes, absolute output valuesfrom the calculations assuming total NW length of 30 micrometers andinner diameter of 100 nm. Left/bottom axes, general dimensions rewrittenas dimensionless ratios. The SA of a straight NW with equivalent volumeis denoted by the dashed line.

In summary, this geometric analysis supports the thermodynamic aspect ofthis model and provides a plausible pathway for surface energyreductions, namely through SA reductions: PSs can have less SA comparedto uniform diameter NWs and NWs with longer pitches can have less SAthan NWs with shorter pitches. Given the energetic anisotropy of Sisurfaces, it is noted that further SE reductions (and deviations fromthis model) likely occur via a relative expansion in SA of lower energyfacets at the expense of higher energy facets. Furthermore, it is notedthat this model and geometric calculations do not make assumptions aboutthe material composition or absolute sizes.

Example 4

To test the validity of this model and to determine the scope andgenerality of PR Crystal Growth, shells were deposited onto NW cores ofdifferent diameters and material compositions. Similar to the growths on100 nm cores described previously, 30 nm diameter Si NW cores were grownby the VLS process in a CVD followed by introduction of SiH₄ at lowpressures for shell growth. SEM images (FIG. 3A) of transferredcore/shell NWs from this synthesis revealed a diameter-modulated PS NWstructure. PS growth on these 30 nm cores yielded a high observedmodulation amplitude of ˜8 with outer shell diameter ˜230 nm, innershell diameter ˜30 nm, and pitch of ˜600 nm; such a high modulationamplitude of diameter-modulated NWs has not been achieved previouslythrough synthetic means.

Moreover, these thinner PS NWs allow for higher resolutioncharacterization compared to the much larger PS NWs shown in FIG. 1. Ahigh-resolution, lattice resolved TEM image (FIG. 3B) obtained at thejunction between one inner and outer shell shows continuous latticefringes over the entire structure with no indications of defects or aninterface between the core and shell. Electron diffraction from thisarea (FIG. 3B, lower inset) again yielded a single set of spots in the[111] zone axis, confirming the highly crystalline and defect-freenature of the small PS NWs. Si deposition on Si NW cores of 50 and 80 nmdiameters at similar PS growth conditions also yieldeddiameter-modulated NWs.

In addition to Si materials, the growth of Ge PS NWs was also explored.Notably, the introduction of GeH₄ at sufficiently low pressures and hightemperatures onto uniform-diameter Ge cores with diameters of 50 nmyielded diameter-modulated PS NWs (FIG. 3C). From these growths, itappears that PR Crystal Growth is possible with cores of differentdiameters and material compositions. This generality is consistent withthe thermodynamic understanding of the above model, where surface energyminimization can occur with periodic crystals growing on any 1Dmaterial.

FIG. 3 shows the generality and scope of PR Crystal Growth in thisexample. FIG. 3A shows SEM images of Si PSs on Si core NWs withdiameters of 30 nm. Scale bars, 1 micrometer. FIG. 3B shows an HRTEMimage of the transition from inner to outer shell of a Si PS grown on a30 nm core NW. Scale bar, 10 nm. Inset, electron diffraction pattern inthe [111] zone axis. FIG. 3C shows an SEM image of Ge PSs deposited on aGe core NW with a diameter of 50 nm. Scale bar, 1 micrometer.

Example 5

This example demonstrates the potential for controlling cross-sectionalaspect ratio through changes in shell growth conditions. SEM imagesrevealed that the deposition of Si onto NW cores at very low flow ratesof SiH₄ generated high (˜4:1) aspect ratio structures (FIG. 3D) comparedto the low (˜1) aspect ratio observed at higher SiH₄ flow rates such asthe one shown in FIG. 1B, right. In particular, FIG. 3D shows a head-onSEM image of a high (˜4:1) aspect ratio Si PS NW grown with low flowrates of SiH₄. Scale bar, 400 nm.

TEM images and diffraction of high aspect ratio structures grown on 100nm Si cores showed similar material quality as well as the same growthdirections as the Si NWs from FIG. 1, indicating that the aspect ratiowas controlled by shell deposition conditions and did not result fromdifferences in the underlying core (e.g. growth direction). It isproposed that the inherent anisotropies of crystal surfaces combinedwith the kinetics of surface diffusion allowed this synthetic tunabilityof aspect ratio. For a given Si surface, the ratio of surface diffusioncoefficients of Si adatoms in different crystalline directions can be ashigh as 1000:1. Moreover, mass transport from one facet to another mayoccur for small Si facet lengths. The dependence of these diffusionalanisotropies on deposition conditions as well as the different relativelengths and energy densities of different surfaces may lead to formationof the different aspect ratios. The PR Crystal Growth model can serve asa foundation for creating even more complex morphologies.

Example 6

Given the broad range of morphologies accessible through PR CrystalGrowth, this example investigated the optical properties of various PSNW structures. Dark field optical images (FIG. 4A) of two PS NWsindicate that distinct wavelengths were scattered efficiently atdifferent points along the axes of the NWs. SEM images of the NW in FIG.4A, left, revealed that the structure has outer and inner diameters of˜250 nm and ˜205 nm, respectively, indicating that the modulation ofscattered light correlates with the modulation of the NW diameter. Theoptical properties of uniform-diameter Si NWs may be significantlyinfluenced by their diameters and cross-sectional morphologies.

The absorption properties of PS NWs was simulated usingfinite-difference time-domain (FDTD) light absorption simulations (FIG.4B). The dimensions for the NW shown in FIG. 4A, left, were used todefine the simulation geometry. Absorption spectra for the inner andouter shells revealed that distinct absorption of light could occuralong the axis of the NW. Distinct optical resonant absorption modeprofiles (FIG. 4B, inset) illustrate the localization of lightabsorption in the inner vs. outer shell for wavelengths noted by 1 (455nm) and 2 (490 nm), respectively. The optical resonant mode at 530 nmwas confined primarily to the outer shell, which leads to ˜3-fold higherabsorption compared to the inner shell.

FIG. 4 shows optical properties of Si PS NWs. FIG. 4A, left and right,Dark field optical images of PS NWs with different dimensions asdetermined from SEM measurements. Inner and outer diameters of the NW inthe left image were used as input dimensions for absorption simulationsin FIGS. 4B and 4C. FIG. 4B shows light absorption of inner vs. outershell. FDTD simulations of transverse-electric light absorption in a SiPS NW with the same dimensions as the NW from FIG. 4A with a 6micrometer pitch. Spectra were obtained from finite-volume slices at thecenter of the outer and inner shells. Inset, absorption mode profiles atwavelengths of 445 nm (top) and 490 nm (bottom) denoted by 1 and 2.

Example 7

This example explores how the absorption properties ofdiameter-modulated NWs may change as a function of pitch. FDTD lightabsorption simulations (FIG. 4C) were performed for a NW whose inner andouter diameters were same as for FIG. 4B, but with shorter pitches. Thetotal (i.e. inner and outer shell) absorption spectra for NWs withpitches of 440 nm and 400 nm reveal high-amplitude peaks at longerwavelengths. FIG. 4C shows light absorption of PS NWs with smallpitches. FDTD simulations of total light absorption in Si PS NWs withpitches of 400 and 440 nm. Inner and outer shell diameters are the sameas in FIG. 4B. Analysis of the mode profiles (FIG. 12) for thesewavelengths indicate that they were grating modes and that the peakposition for a given mode red shifted as pitch increases. Takentogether, the dark field optical images and absorption simulationsrevealed that the optical properties are tunable through controlled PRCrystal Growth.

FIG. 12 shows grating mode profiles. FIG. 12A shows absorption modeprofiles corresponding to peaks in FIG. 4C. Profiles are from 782 nm(left) and 929 nm (right) for a PS NW with outer diameter of 250 nm,inner diameter of 205 nm, and pitch of ˜440 nm. Scale bars, 200 nm. FIG.12B shows absorption mode profiles at 755 nm (left) and 892 nm (right)for a PS NW with outer diameter of 250 nm, inner diameter of 205 nm andpitch of 400 nm. Scale bars, 200 nm.

The following discussion gives perspective on PR Crystal growth comparedto other synthetic techniques and clarifies the relationship to anddistinctions from traditional PR instability. Thermodynamically, for PRinstability, any sinusoidal surface perturbation (i.e.diameter-modulation) that develops along the axis of the cylinder with acharacteristic wavelength greater than the circumference of the originalcylinder will lower the surface energy and thus will grow in time.However, it has been proposed that the resultant spacing of 0D particlesis determined by the kinetics; the fastest growing perturbation is ˜4.5times the diameter of the cylinder, which eventually pinches off fromthe cylinder, preventing subsequent mass transfer and the attainment oflower energy configurations with longer pitches. Unlike PR instabilitywhere the source of mass transfer comes from the original 1D materialitself, the source for PR Crystal Growth is provided externally fromdecomposition of a reactant (e.g. SiH₄ or GeH₄); thus, the kinetics ofadatom diffusion, nucleation and growth along the axis of the NW can becontrolled, for example, by increasing temperature without rearrangingthe underlying core and without the irreversible pinch off that canprevent subsequent mass transfer as it does for cases of constant-volumetransformations (i.e. PR of NW cores, or PR of heterostructured shellson cores).

In summary, PR Crystal Growth is general to 1D materials as this modelsuggests, and with experimental demonstration on Si and Ge cores ofdifferent diameters. As various material properties (e.g. thermal,electrical, mechanical) of NWs depend on the NW diameter,cross-sectional morphology, and composition, PR Crystal Growth has mayallow for novel functionality in NW devices for a variety ofapplications.

Example 8

The following are various materials and methods used in the aboveexamples. Si core NWs were synthesized as described previously via theAu-catalyzed VLS mechanism. See, e.g., U.S. Pat. No. 7,211,464;7,301,199; 7,476,596; 7,595,260; 7,666,708; 7,915,151; or 8,153,470,each incorporated herein by reference. Following core growth, thefurnace temperature was ramped to 700 to 850° C. for PS growth. At thistemperature, shells were grown for 1-60 minutes at ˜0.2 torr with gasflow rates of 0.15-10 sccm SiH₄ and 0-200 sccm H₂. For some syntheses,diborane (B₂H₆, 100 p.p.m. in H₂) or phosphine (PH₃, 1000 p.p.m. in H₂)was introduced to the reactor during shell growth at 0.5-20 sccm flowrates. Germanium core NWs were typically synthesized from 50 nm Aucatalysts at a total pressure of 300 torr with 200 sccm H₂ and 20 sccmgermane (GeH₄; 10% in H₂) flow rates. Ge cores were nucleated for 5minutes at 330° C. and grown for another 50 minutes at 270° C. To growGe PSs, the temperature was increased to 450° C. and the pressuredecreased to ˜0.2 torr with 10% GeH₄ flow rates of 30 sccm.

End-on view SEM images of PS NWs were recorded directly from theas-synthesized growth wafers. For plan view SEM images and NW pitchmeasurements, NWs were transferred to Si₃N₄-coated Si wafers. For TEM,STEM, and diffraction analysis, NWs were transferred to an amorphouscarbon-coated copper TEM grid. EDS peaks were assigned by the PeakIDalgorithm in the EDAX Genesis software and confirmed by checkingstandard references (EDAX). EELS data were collected by fixing theconverged electron beam at various points along the NW and spatiallyseparating the transmitted electrons using an energy filter in the STEMcolumn. Energy loss vs. position on the CCD was calibrated using thezero-loss peak. Spectra were collected and added 1000 times each.

Optical dark field images were recorded of NWs which were transferred tosilicon nitride-coated substrates. FDTD calculations assumed plane waveswith either transverse-electric (TE) or transverse-magnetic (TM)polarization states were vertically incident on single NWs on 200 nmthick Si₃N₄/100 nm thick SiO₂ substrates. The absorption cross sectionof a NW was calculated by integrating J·E at each grid point over oneoptical period, where J and E are the polarization current density andelectric field, respectively. For a PS NW with a finite pitch size, theNW was divided into small segments with lengths of 200 nm along the NWaxis and calculated the absorption cross section at each segment (FIG.4B). In the PS NW simulations, calculations were performed with aspatial resolution of 10/sqrt(3), 10, and 10 nm for each axis and acalculation domain size of 1.6/sqrt(3)×6×0.9 micrometers³.

Surface areas of PS NWs were calculated by constructing a unit cellcomprising adjoining cylinders of lengths L_(inner) and L_(outer) anddiameters D_(inner) and D_(outer); one inner and one outer cylinderdefine a unit cell (FIG. 13A). FIG. 13A shows a schematic of the unitcell used in the surface area calculation model. For geometric SAcalculations, a unit cell of a PS NW was approximated as two adjacentcylinders of lengths L_(inner) and L_(outer) and diameters D_(inner) andD_(outer). It was assumed that the additional volume added, V_(added),to the system was contained within the outer shell. The geometry wassystematically varied (either modulation amplitude as in FIG. 3D orpitch as in FIG. 3D) to calculate the lowest surface area configuration.Considerations of multiple V_(added)s are found in FIG. 11.

Periodic Shell Nanowire (PS NW) Synthesis. Si PS NW growth: Au catalysts(30, 50, 80, and 100 nm; BBI International via Ted Pella) were dispersedon an oxidized Si wafer functionalized with 10% poly-L-lysine (SigmaAldrich). After rinsing in DI water and drying with nitrogen, thesubstrates were placed into a home-built chemical vapor depositionreactor and the system was evacuated to a base pressure of ˜5 mTorr. Sicores were grown via the Au-catalyzed VLS mechanism for ˜10-60 minutesat 465° C. and a total pressure of 40 torr with 1 sccm silane (SiH₄;100%) and 60 sccm hydrogen (H₂) flow rates. Following core growth, thefurnace temperature was ramped to 700-850° C. for shell growth. At thistemperature, shells were grown for 1-60 minutes at ˜0.2 torr with gasflow rates of 0.15-10 sccm SiH₄ and 0-200 sccm H₂. For some syntheses,diborane (B₂H₆, 100 p.p.m. in H₂) or phosphine (PH₃, 1000 p.p.m. in H₂)were introduced to the reactor during shell growth at 0.5-20 sccm flowrates.

Ge PS NW growth: Germanium core NWs were typically synthesized from 50nm Au catalysts at a total pressure of 300 torr with 200 sccm H₂ and 20sccm germane (GeH₄; 10% in H₂) flow rates. Ge cores were nucleated for 5minutes at 330° C. and grown for another 50 minutes at 270° C. To growGe PSs, the temperature was increased to 450° C. and the pressuredecreased to ˜0.2 torr with 10% GeH₄ flow rates of 30 sccm.

NW Characterization. End-on view scanning electron microscope (SEM,Zeiss Ultra Plus field emission SEM) images of PS NWs were recordeddirectly from the as-synthesized growth wafers. For plan view SEM imagesand NW pitch measurements, NWs were transferred to Si₃N₄-coated Siwafers. For transmission electron microscope (TEM) and diffractionanalysis, NWs were shear transferred to an amorphous carbon-coatedcopper TEM grid and imaged directly in JEOL 2100 or JEOL 2010F fieldemission high resolution TEMs operating at 200 keV. Scanning TEM (STEM)imaging and analysis was performed on an energy filtered C_(s)-STEMAberration Corrected Zeiss Libra 200 MC operating at 200 keV equippedwith pre- and post-filter high-angle annular dark-field (HAADF)detectors, dual x-ray detectors for energy dispersive x-ray spectroscopy(EDS), and drift correction. The system was tuned to 18 mrad informationtransfer with a beam spot size less than 2 nm. EDS spectra werecollected by fixing the converged electron beam at various points alongthe NW for 5 minutes and recording the resultant x-rays with 400microsecond dwell time and 102.4 microsecond amp time. EDS peaks wereassigned by the PeakID algorithm in the EDAX Genesis software andconfirmed by checking standard references (EDAX). EDS maps were recordedfor ˜4 hours in cropped regions of interest at 512×400 resolution, 400microsecond dwell time per pixel, and 102.4 microsecond amp time.Electron energy loss spectra (EELS) were collected by fixing theconverged electron beam at various points along the NW and spatiallyseparating the transmitted electrons (19.50 eV/micrometers) using theenergy filter. Energy loss vs. position on the CCD was calibrated usingthe zero-loss peak. Spectra were collected and added 1000 times each.

Optical dark field images of NWs on silicon nitride-coated substrateswere recorded on an Olympus BX51 microscope.

FDTD calculations. Plane waves with either transverse-electric (TE) ortransverse-magnetic (TM) polarization states are simulated to bevertically incident to a single NW on 200 nm thick Si₃N₄/100 nm thickSiO₂ substrate. Periodic boundary conditions and perfectly matchedlayers were applied along the NW axis and at the other boundaries,respectively. The total-field scattered-field (TFSF) method was appliedto project an infinite plane wave to a NW. The Drude-critical pointsmodel was used to model the dispersive properties of Si NWs over thewavelength range of 400-800 nm. The absorption cross section of a NW wascalculated by integrating J·E at each grid point over one opticalperiod, where J and E are the polarization current density and electricfield, respectively. For a PS NW with a finite pitch size, the NW wasdivided into small segments with lengths of 200 nm along the NW axis andcalculated the absorption cross section at each segment (FIG. 4B). Inthe PS NW simulations, calculations were performed with a spatialresolution of 10/sqrt(3), 10, and 10 nm for each axis and a calculationdomain size of 1.6/sqrt(3)×6×0.9 m³.

Surface areas and energies of PS NWs. This analysis compared the surfaceareas of various 1D configurations, all of which have the same totalvolume. By approximating the total surface energy of a 1D material withits total surface area and its cross-sectional morphology ascylindrical, it was proposed that PSs could have reduced surfaceenergies compared to uniform diameter NWs of equivalent volume. Withthis straightforward geometric analysis, the model wasmaterial-independent and dimensionless. The assumptions and treatment ofthe model are discussed below.

Surface area approximation for surface energy. For a solid withenergetically anisotropic surfaces, the total surface energy is

$\begin{matrix}{\gamma_{Total} = {\sum\limits_{i}{\gamma_{i}A_{i}}}} & (1)\end{matrix}$

where γ_(Total) is the total surface energy of the solid, γ_(i) is thesurface energy density of facet i, and A_(i) is the area of surface i.To a first approximation, the relative surface energies of different 1Dstructures can be approximated with their total surface areas (SAs),assuming an energetically isotropic solid. For this approximation to bereasonable, the surface energy densities should not differsignificantly. For commonly observed silicon surfaces, experimentalmeasurements have yielded surface energy densities to be within ˜15% ofeach other; specifically the surface energy densities of Si {100},{110}, {111}, and {113} surfaces are estimated to be 1.36-1.40,1.43-1.51, 1.23-1.34, and 1.38 Jm⁻².

In principle, the inclusion of specific surface energy densities andnon-circular cross section would more accurately represent thesestructures. However, doing so would unnecessarily introduce adjustableparameters at present. First, experimental and theoretical techniquesyield a range of different surface energy density values. In addition tothe disputed surface energy densities, the exact identity and nature(e.g. reconstructed vs. not reconstructed, presence and identity ofadsorbates, etc.) of the many facets on the PS NWs are not known. Thus,incorporating either the surface energy density values or specificsurface identities to the model would unnecessarily complicate a ratherstraightforward approach and limit the model's generality for othermaterials. This geometric approach makes no other specific assumptionsabout Si and can thus be extended to other 1D materials where the rangeof surface energy densities for various surfaces are reasonably close.

Surface area comparisons for various structures with fixed volume. Thesurface areas of various 1D configurations were compared, all of whichhave the same total volume. For PS growth, the core itself does notchange, and so all of the volume is added to the shell. The PS NW wasapproximated to be formed of adjoining cylinders of lengths L_(inner)and L_(out), and diameters D_(inner) and D_(outer); one inner and oneouter cylinder define a unit cell (FIG. 13A). The additional volumeadded, V_(added), to t the system was assumed to be contained within theouter shell and is, thus, expressed as

$\begin{matrix}{V_{added} = {{V_{outer} - V_{core}} = {\frac{\pi \; L_{outer}}{4}\left( {D_{outer}^{2} - D_{core}^{2}} \right)}}} & (2)\end{matrix}$

Furthermore, it was assumed that the diameter of the inner shell as thatof the core NW, which is reasonable for short growth times.

Surface area of PS with constant volume and fixed pitch. The surfaceareas of the following structures were compared, all of which haveequivalent V_(added) shell volumes: uniform diameter NWs and various PSNWs with different diameter modulations and assuming the inner shelldiameter is constant (see FIG. 11, right for a schematic representationof various configurations for constant V_(added)). In FIG. 2C, thespecific case in which V_(added) is constant and pitch is fixed at 3micrometers but modulation amplitude can change was considered.V_(added) was obtained from dimensions of a PS NW measured by SEM (FIG.10; L_(inner)=1,500 nm, L_(outer)=1,500 nm, D_(inner)=100 nm,D_(outer)=300 nm, and total length, L_(total),=30 micrometers). Forthese dimensions, V_(added)=9.4×10⁸ nm³ for the 30 micrometer NW. Ifthis V_(added) were distributed uniformly along the NW axis, the NWwould have a uniform diameter along its axis of:

$\begin{matrix}{D_{{straight},{equiv}} = \left( \frac{4\left( {V_{added} + V_{core}} \right)}{\pi \left( {L_{inner} + L_{outer}} \right)} \right)^{\frac{1}{3}}} & (3)\end{matrix}$

In the case of the above geometry, the equivalent diameter for astraight NW would be D_(straight, equiv)=˜223.6 nm.

Next, the SA of this PS NW was compared to the SA of the straight NWwith equivalent volume. The SA of the PS within one unit cell is the sumof the SAs of the inner shell cylinder and the outer shell cylinder(excluding surfaces which are shared at the interface of the twocylinders):

$\begin{matrix}{{SA}_{PS} = {\pi \left\lbrack {\left( {D_{inner}L_{inner}} \right) + \left( {L_{outer}D_{outer}} \right) + \left( {\frac{1}{2}D_{outer}^{2}} \right) - \left( {\frac{1}{2}D_{{inner}\;}^{2}} \right)} \right\rbrack}} & (4)\end{matrix}$

And the SA of the straight NW with equivalent volume is:

SA_(straight,equiv.) =πD _(straight,equiv.)(L _(inner) +L _(outer))  (5)

For these two configurations, the SA of the PS to the SA of the straightNW is SA_(PS)/SA_(straight,equiv.).

Next, the SAs of NWs with different modulation amplitudes was determinedby varying L_(outer) (and thus L_(inner), sinceL_(outer)+L_(inner)=pitch=3,000 nm) for 20 nm<L_(outer)<3,000 nm inincrements of 20 nm. Since V_(added) is constant, D_(outer) must alsochange with L_(outer). Rearranging equation (2) for D_(outer) yields:

$\begin{matrix}{D_{outer} = \left( {\frac{4V_{added}}{\pi \; L_{outer}} + D_{inner}^{2}} \right)^{\frac{1}{2}}} & (6)\end{matrix}$

For every D_(outer), SA was calculated. Absolute SA values can becompared for a given D_(outer) or dimensionless comparisons can be madewith ratio of PS SA to Straight NW SA for a given diameter modulation(where diameter modulation=D_(outer)/D_(inner), and D_(inner) isconstant here at 100 nm). (See FIG. 11A for additional calculations ofSA vs. diameter modulation with different values of V_(added) andto-scale representations of various modulation amplitudes and theircorresponding SAs.)

Surface area of PS with constant volume and varying pitch. For the abovecalculations, V_(added) was constant, the pitch was fixed and SA wascalculated as function of modulation amplitude. Given that differentpitches were observed experimentally, it is also important to comparehow SA changes for a given constant volume if pitch is not a fixedparameter. For consideration of SA vs. pitch (FIG. 2D), the abovesimulation was expanded by considering pitches ranging from 500 nm to 15micrometers in increments of 100 nm. For the above calculation, a totalNW length of L_(total)=30 micrometers was assumed such that there are 10unit cells per NW for a pitch of 3 micrometers; this allowed absolutesurface area comparisons for a length that is comparable to the NWsemployed. However, for the general case with different pitches or totallengths, there are n=L_(total)/pitch unit cells per NW and the volumewas distributed evenly among the n outer shells. For each pitch, theabove calculations were performed to produce SA values as a function ofdiameter modulation. The minimum SA for a given pitch is then plotted asa function of SA vs. pitch. (See FIG. 11B for additional calculations ofSA vs. pitch with different values of V_(added) and to-scale schematicsof the various pitches, diameter modulations, and SAs.)

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

It should also be understood that, unless clearly indicated to thecontrary, in any methods claimed herein that include more than one stepor act, the order of the steps or acts of the method is not necessarilylimited to the order in which the steps or acts of the method arerecited.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

What is claimed is:
 1. An article, comprising: a nanowire comprising acontinuous core comprising a first longitudinal portion comprising ashell at least partially surrounding the core and having a substantiallyconstant cross-sectional area, and a second longitudinal portion havinga substantially constant cross-section area smaller than the firstlongitudinal portion, wherein the first longitudinal portion has a firstdimension orthogonal to the core and a second dimension orthogonal tothe first dimension and to the core, wherein an aspect ratio of thefirst dimension to the second dimension is at least about 1.5:1, andwherein the core and the shell material have different compositions. 2.The article of claim 1, wherein the core comprises a semiconductor. 3.The article of any one of claim 1 or 2, wherein the core comprises ametal.
 4. The article of any one of claims 1-3, wherein the corecomprises a dielectric.
 5. The article of any one of claims 1-4, whereinthe core comprises a semiconductor oxide.
 6. The article of any one ofclaims 1-5, wherein the core comprises SiO₂.
 7. The article of any oneof claims 1-6, wherein the shell comprises a plurality of discontinuesshell regions.
 8. The article of claim 7, wherein the plurality ofdiscontinuous shell regions are substantially regularly longitudinallyspaced.
 9. The article of claim 8, wherein the plurality ofdiscontinuous shell regions have a periodicity of less than about 20micrometers.
 10. The article of any one of claim 8 or 9, wherein theplurality of discontinuous shell regions have a periodicity of greaterthan about 200 nm.
 11. The article of any one of claims 8-10, whereinthe plurality of discontinuous shell regions have a periodicity ofgreater than about 500 nm.
 12. The article of any one of claims 7-11,wherein the discontinuous shell regions are each separated by at least 5nm.
 13. The article of any one of claims 7-12, wherein the discontinuousshell regions comprise a longitudinal portion having a length of atleast 5 nm and a substantially constant cross-sectional area.
 14. Thearticle of claim 13, wherein the discontinuous shell regions are eachseparated by a longitudinal portion of the nanowire having a length ofat least 5 nm.
 15. The article of claim 14, wherein the discontinuousshell regions further comprises a transitional portion having a lengthof at least 10 nm between the longitudinal portion and the longitudinalportion of the nanowire.
 16. The article of any one of claims 7-15,wherein at least one of the discontinuous shell regions consists of asingle crystal.
 17. The article of any one of claims 1-6, wherein theshell is continuous.
 18. The article of any one of claims 1-17, whereinthe second longitudinal portion of the nanowire comprises the core andthe shell.
 19. The article of any one of claims 1-18, wherein the firstlongitudinal portion of the nanowire has a maximum dimension, orthogonalto the core, of less than about 1 micrometer.
 20. The article of any oneof claims 1-19, wherein the first longitudinal portion of the nanowirehas a maximum dimension, orthogonal to the core, of less than about 500micrometers.
 21. The article of any one of claims 1-21, wherein thefirst longitudinal portion of the nanowire has a maximum dimension,orthogonal to the core, of less than about 250 micrometers.
 22. Thearticle of any one of claims 1-21, wherein the second longitudinalportion of the nanowire has a maximum dimension, orthogonal to the core,of less than about 150 micrometers.
 23. The article of any one of claims1-22, wherein the shell comprises Si.
 24. The article of any one ofclaims 1-23, wherein the shell consists essentially of Si.
 25. Thearticle of any one of claims 1-24, wherein the shell comprises asemiconductor.
 26. The article of any one of claims 1-25, wherein theshell comprises a metal.
 27. The article of any one of claims 1-26,wherein the shell comprises a dielectric.
 28. The article of any one ofclaims 1-27, wherein the shell comprises a semiconductor, and the corecomprises a semiconductor, wherein the core and the shell semiconductorsare distinguishable.
 29. The article of any one of claims 1-28, whereinthe core has an average diameter of less than about 1 micrometer. 30.The article of any one of claims 1-29, wherein the core has an averagediameter of less than about 300 nm.
 31. The article of any one of claims1-30, wherein the core has an average diameter of less than about 100nm.
 32. The article of any one of claims 1-31, wherein the core has avariation in average diameter of less than about 20%.
 33. The article ofany one of claims 1-32, wherein the nanowire has a maximum dimension,orthogonal to the core, of less than about 1 micrometer.
 34. An article,comprising: a nanowire comprising a continuous semiconductor oxide coreand a plurality of discontinuous semiconductor shell regions.
 35. Thearticle of claim 34, wherein the core comprises SiO₂.
 36. The article ofany one of claim 34 or 35, wherein the plurality of discontinuoussemiconductor shell regions are substantially regularly longitudinallyspaced.
 37. The article of claim 36, wherein the plurality ofdiscontinuous semiconductor shell regions have a periodicity of lessthan about 20 micrometers.
 38. The article of any one of claim 36 or 37,wherein the plurality of discontinuous semiconductor shell regions havea periodicity of greater than about 500 nm.
 39. The article of any oneof claims 34-38, wherein the discontinuous semiconductor shell regionsare each separated by at least 5 nm.
 40. The article of any one ofclaims 34-39, wherein the discontinuous semiconductor shell regionscomprise a longitudinal portion having a length of at least 5 nm and asubstantially constant cross-sectional area.
 41. The article of claim40, wherein the discontinuous semiconductor shell regions are eachseparated by a longitudinal portion of the nanowire having a length ofat least 5 nm.
 42. The article of claim 41, wherein the discontinuoussemiconductor shell regions further comprises a transitional portionhaving a length of at least 10 nm between the longitudinal portion andthe longitudinal portion of the nanowire.
 43. The article of any one ofclaims 34-42, wherein at least one of the discontinuous semiconductorshell regions comprises a first dimension orthogonal to the core and asecond dimension orthogonal to the first dimension and to the core,wherein an aspect ratio of the first dimension to the second dimensionis at least about 1.5:1.
 44. The article of any one of claims 34-43,wherein at least one of the discontinuous shell regions consists of asingle crystal.
 45. The article of any one of claims 34-44, wherein theplurality of discontinuous semiconductor shell regions comprises Si. 46.The article of any one of claims 34-45, wherein the plurality ofdiscontinuous semiconductor shell regions consists essentially of Si.47. The article of any one of claims 34-46, wherein the core has anaverage diameter of less than about 1 micrometer.
 48. The article of anyone of claims 34-47, wherein the core has a variation in averagediameter of less than about 20%.
 49. The article of any one of claims34-48, wherein the nanowire has a maximum dimension, orthogonal to thecore, of less than about 1 micrometer.
 50. The article of any one ofclaims 34-49, wherein the core is solid.
 51. The article of any one ofclaims 34-49, wherein the core is hollow.
 52. An article, comprising: ananowire comprising a continuous semiconductor oxide core comprising afirst longitudinal portion having a length of at least 5 nm and a firstsubstantially constant cross-sectional area, a second longitudinalportion having a length of at least 5 nm a second substantially constantcross-section area smaller than the first longitudinal portion, and atransitional portion between the first longitudinal portion and thesecond longitudinal portion, the transitional portion having a length ofat least about 10 nm.
 53. The article of claim 52, wherein the corecomprises SiO₂.
 54. The article of any one of claim 52 or 53, whereinthe plurality of discontinuous semiconductor shell regions aresubstantially regularly longitudinally spaced.
 55. The article of claim54, wherein the plurality of discontinuous semiconductor shell regionshave a periodicity of less than about 20 micrometers.
 56. The article ofany one of claims 54-55, wherein the plurality of discontinuoussemiconductor shell regions have a periodicity of greater than about 500nm.
 57. The article of any one of claims 52-56, wherein at least one ofthe discontinuous semiconductor shell regions comprises a firstdimension orthogonal to the core and a second dimension orthogonal tothe first dimension and to the core, wherein an aspect ratio of thefirst dimension to the second dimension is at least about 1.5:1.
 58. Thearticle of any one of claims 52-57, wherein at least one of thediscontinuous shell regions consists of a single crystal.
 59. Thearticle of any one of claims 52-58, wherein the plurality ofdiscontinuous semiconductor shell regions comprises Si.
 60. The articleof any one of claims 52-59, wherein the plurality of discontinuoussemiconductor shell regions consists essentially of Si.
 61. The articleof any one of claims 52-60, wherein the core has an average diameter ofless than about 1 micrometer.
 62. The article of any one of claims52-61, wherein the core has a variation in average diameter of less thanabout 20%.
 63. The article of any one of claims 52-62, wherein thenanowire has a maximum dimension, orthogonal to the core, of less thanabout 1 micrometer.
 64. A method, comprising: depositing a shellmaterial on a nanowire by flowing a fluid comprising the shell materialor a precursor thereof over the nanowire at a flowrate of less thanabout 10 sccm at a temperature of between about 700° C. and about 900°C. and under a pressure of less than about 1 torr.
 65. The method ofclaim 64, comprising flowing the fluid longitudinally over the nanowire.66. The method of any one of claim 64 or 65, wherein the precursorcomprises SiH₄.
 67. The method of any one of claims 64-66, wherein theprecursor comprises GeH₄.
 68. A method, comprising: depositing a shellmaterial on a nanowire by flowing a fluid comprising the shell materialor a precursor thereof over the nanowire such that the surface diffusionlength of the shell material on the surface of the nanowire is at leastabout 100 nm.
 69. The method of claim 68, comprising flowing the fluidlongitudinally over the nanowire.
 70. The method of any one of claim 68or 69, wherein the precursor comprises SiH₄.
 71. The method of any oneof claims 68-70, wherein the precursor comprises GeH₄.
 72. An article,comprising: a nanowire comprising a continuous core and a plurality ofdiscontinuous shell regions, wherein the discontinuous shell regionseach consist of a single crystal.